Three Hearts and Three Lions.
  The very first entry on the table of inspirational and educational reading,
  Appendix: N of the first edition AD&D Dungeon Master's Guide -
  penned by Gygax himself. Admittedly self-professed to be ordered by alpha, the
  significance of Poul Anderson's influence on Gygaxian Dungeons & Dragons
  can't be understated: perhaps in contention with Jack Vance in representing
  the single most influential piece of the original game. While Vance's work has
  persisted even through the current edition in the form of his
  Dying Earth's interpretation on the invocation of magic - Anderson's
  influence on the undertone of the system - on monsters, on alignment, on
  expectations of a party member as player: all things that differ between the
  perspectives of a modern role-player and an old-school one - can't be
  understated; and the role of this, only one of dozens of volumes he wrote, in
  providing that context - in bridging that gap - shouldn't be underestimated.
  From one role-player to another, one who came to the hobby in the
  post-Gygaxian era and who has since come to embrace the traditions enshrined
  before, I present one perspective: Three Hearts and Three Lions - and
  why it should be the first thing that new-comers to OSR gaming should read.
The Yarn, Itself
  Three Hearts and Three Lions is a quick read - 220 digest sized pages
  in my copy - and largely a pleasant one. The tone of the book is jocular - told
  as though a friend was telling it to you at a bar and as though you're not
  sure whether or not to believe, but it's fun enough that you wind up
  listening. There are moments where the diction is difficult to follow -
  namely, when accents are affected by spelling: or when particular references
  are made to material science - however at no point in the reading did I have
  to go over the same text more than a second time. 
  Likewise - I found myself getting attached to the characters: when something
  went ill for one, or when something happened to threaten the direction that I
  had wanted the story to take, I was personally affronted: fighting through
  temptation to skip ahead to see the resolution - in all but one case, my
  admonitions of the plot being rebuffed - again, as a friend might turn the
  twist and slam a mug against the table: pleasing the listener at the
  unexpected turn of events that led to where you wanted them to lead in the
  first place. 

Fanciful language is somewhat lacking - though fanciful elements are not: and
  each chapter leads into the next as episodes of the larger tale. You need to
  know, for example, the characters that are met in the first few chapters - but
  later chapters, such as the mystery of the lycanthrope, could stand on their
  own as a pulp publication outside the book - their inclusion serving as a way
  to bemuse the reader, but also to explain certain elements of continuity in
  the story and in the setting which otherwise might go unexplained: an act that
  the author is aghast to perform! Seemingly unrelated depictions, seemingly
  unconnected events, can be tied and understood to contribute to an over-all
  atmosphere and to better convey the nature of the world to the reader in
  interesting ways, such that these details are absorbed by osmosis rather than
  through a dump of
  
fluff or flavor.
It is not, however, a perfect book.
  To speak to the pieces of the book I did not like, I'll have to spoil a bit -
  I will try to hide the spoiler under a detail tag - which will hide or show at
  your leisure, provided you have a modern browser to use - however, if the tag
  doesn't work, skip ahead to the next heading,
  The Influence on Alignment.
  Were I to complain about the book,
  
    ...I would have to complain about the ending. At the end of the hero's
    journey, he finds out who he truly is: his identity having been hidden from
    him by foul magics. United with his sword and his companions, the author
    reveals he is truly Oiger le Danois! A
    folk hero of European Christendom, hints dropped throughout the text hinting at his identity (including his
    name, itself!) can be pieced together to support the big reveal!
  
  
    The book then concludes - no more action, no more dialog, no more adventure
    in the fanciful alternate Earth - with the equivalent of "He rode off,
    everyone loved him, and he conquered his enemies!" It's as though the point
    of the story was to present the puzzle, daring the reader to figure out who
    the character was - it's presented immediately as his own problem: who am I
    in this world? - before the jig is up and the secret is revealed in the
    finale. It's as though the book was not written to tell the story, but
    instead to masturbate the intellect and education of the author and his
    readers - not to enthrall and enchant, as to that point, it truly does.
  
  
    Similarly, an afterword is provided wherein - not only did the story end
    unceremoniously, but the character
    is transported back to modern times through no effort of his own! In
    a very John Carter of Mars moment, he is transported back into the
    modern world while in the midst of battle - when the iron rails of plot
    necessitates it happen. John Carter transitions to Barsoom on death - and
    Holger Dankse (presumably named after the
    resistance group
    he serves to join) is transported to the alternative timeline when shot in
    the head by the Nazis. Following this, he is said to be seeking grimoires to
    restore him to the alternate Earth, to reunite with those whom he loves: and
    the author says, "I never heard from him again - people do go disappearing
    once in a while, you know" - as though to dare the reader to draw their own
    conclusion that Holger does indeed make it home.
  
  
    Perhaps, it serves as a way to lend credence to the premise - that the story
    is told to the author by a friend who experienced it: so a return to our
    timeline is necessary to fulfill it.
  
  
    And perhaps, I choose to believe he did make it back: as I would
    assume the author desires me to believe - thinking of the fondness I've
    developed, having experienced the adventure with its characters.
  
  
    But in any case - I was disappointed by the ending - twofold: first, in the
    sudden "Everything seems black and impossible... but everything is suddenly
    fine now!" and second, in the intentional ambiguity of the post-script.
  
 
  This point of contention may bother you too - or it may not. I should think
  anyone a fan of adventure and of fantasy would enjoy the story, regardless:
  and may simply do as I have done and chosen to believe.
The Influence on Alignment
  Foremost among the lessons of Three Hearts and Three Lions might be the
  origins of Law and Chaos in the D&D alignment system. 
  But CWR - I thought Michael Moorcock was the source of the Law and Chaos
    alignment spectrum in D&D?
  Many people do - and he did have an influence. His Hawkmoon and
  Stormbringer titles are listed in the same inspirational reading block
  as are Anderson's. However, Moorcock himself
  cites Anderson as an influence and inspiration for his interpretation of
    cosmic order: and though I swear I have read at some point that Gygax, himself, preferred
  Andersonian alignment over Moorcockian, I cannot find a source to cite for it
  and will have to be satisfied pointing out that Anderson was active earlier -
  1961 (Three Hearts and Three Lions), 1954 (The Broken Sword),
  1960 (The High Crusade) - than was Moorcock - 1965
  (Stormbringer), 1963 (Stealer of Souls), 1967 (The Jewel in the Skull
  - the first title in the Hawkmoon series).
But I digress.
  I won't quote Anderson on it - there are a thousand blog results who do when
  you
  google it
  - but where both authors will speak to alignment in cosmic terms: Anderson in
  particular does a good job of divorcing alignment from personality. A good
  person can be Chaotic - although their actions and their activities will
  undermine civilization; simultaneously, a bad person can be Lawful - they may
  murder, pillage, or make war on things that are holy, but those self-serving
  motives, those secular actions: they aren't indicators of their position in
  the greater scene. Anderson clearly and blatantly separates alignment from the
  personalities and goals of the people who serve it: which Moorcock does
  somewhat, although Moorcock tends to infuse his writing with his own
  nihilism, which dilutes the literary concept with intended metaphor. For that
  reason, Anderson is better in this regard - and I will speak no more to
  condemn one author in preference for another.
  Another interesting component of Anderson's alignment: for humans, alignment
  is a choice - though naturally, humans tend towards Law. For other races -
  Elves, Trolls, etc - there isn't a choice. Their very natures are
  infused with Chaos: again, regardless of their goals or motives. Trolls in
  Three Hearts and Three Lions - or, True Trolls from Chainmail -
  and though they may be fickle, they may wage war on the forces of Law, they
  are equally facile in waging diplomacy: something that has more presence in
  The Broken Sword where the Elves do business with the Dwarves, a race
  which can touch iron and which are not born with an innate Chaos nor Law to
  them.
  This bioessentialism - as it may be - is contrasted to the races of men: Elves
  are by their nature Chaotic - but no men are. We see two Lawful men, men who
  were once enemies on the battlefield as a result of ethnic and religious
  differences - but who became brothers with time. This fellowship, this kindred
  spirit of humanity, is a stark answer to the criticism that "orcs can't be
  evil" - absolutely: yes, they can - because like
  Three Hearts and Three Lions, humanity is one race, one people, with
  different creeds and different appearances - each, however, connected together
  by a permeating connection to Law, but given liberty to choose our own paths.
  Once this concept - and that of alignment being something bigger, a dichotomy
  and narrative mechanic to facilitate non-combat interactions between opposing
  factions - than the personality litmus test it has become: once this concept
  has been grasped, other concepts similarly fall into place.
The Influence on Encounters
  But what then - tangibly - can be ascribed to
  Three Hearts and Three Lions that made it into the print of the books?
  Most obviously - monsters.
  As one familiar with the LBBs (reprints, of course - gamer on a budget), when
  reading through Three Hearts and Three Lions, my
  Book II: Monsters and Treasure alarm went off several times during the
  read - it was exciting for me, seeing things in the source material that, in
  the original edition, so obviously paralleled in the game world! Dragons and
  barbarians notwithstanding - as surely, these creatures exist in a multitude
  of sources - other creatures seem to be uniquely inspired by Anderson's work.
  In illustrating as such, I submit two examples - which, in so doing,
    may entail spoilers to the plot: so I will, again, attempt to hide the core
  contents:  
  The Troll
  
    The Troll! The most iconic D&D monster outside the product identifying
    Beholder! Reviewing my copy of Book II: Monsters & Treasure,
    lavishly printed from PDF sparing no expense on my 5 year old HP 4000
    series:
  
  
  
  
  
    Juxtapose this to the penultimate encounter of Holger, et al, in
    Three Hearts and Three Lions:
  
  
  
  
  
    The creatures described are the same - and the creature illustrated for
    D&D has been of this likeness ever since. The band goes on to strike it,
    again and again, only to find it regenerates after each assault:
  
  
  
  
    From there, it's only by accident when the Swan May strikes out with the
    torch do they discover the creature's weakness to flame - rapidly (through
    player character ingenuity) turning the tide of the
    encounter. 
  
  
  
  
  
    In either case - if you have not read Three Hearts and Three Lions,
    the troll scene is truly evocative: it's one to take inspiration from, in
    terms of how to play trolls when encountered in the D&D wild, and one
    that - for players who have not experienced trolls before (or even some who
    have) would provide a terrifying and memorable experience.
  
 
  The Nixie
  
    While less dramatic than the troll - the Nixie: an underwater fey who
    captures Holger with intent to hold him captive and prevent his destiny - is
    an interesting addition to this list as it translates directly into the
    rulebook at the table. Consider Book II: Monsters & Treasure:
  
  
  
  
  
    Each of these elements, we see referenced in
    Three Hearts and Three Lions as our protagonist wrestles with the
    lake spirit. Firstly, regarding the mechanism of imprisonment of a targeted
    character...
  
  
  
  
    ...and secondly, regarding the guardians of the Nixie - down to the named
    species! 
  
  
  
    When Holger escapes from the predicament (more to come on that escapade
    later in the article) he does so using the mechanism described - namely,
    employing a flaming weapon to fend the pike off while swimming away from the
    Nixie's grotto: 
  
  
  
    
   
  
  Here found, admittedly, is a point of diversion between the book and the game.
  In Three Hearts and Three Lions, the fey cannot abide iron and cannot
  abide ultra-violet radiation (or, daylight - hence, you see the fey at night
  in their mythological tales of origin) - as such, the Nixie is held at bay by
  the burning dagger: in D&D, presumably, the flaming aspect must have been
  too accessible to make gamist sense. That said - the other parallels are too
  close to be coincidental: and the reader - again - is presented with a
  situation where wits are required to escape: something to challenge a party
  with and an example of how, using an effect like Charm Person - which,
  in early editions, was extremely hard to deter once it's set in - what appears
  to the modern gamer to be certain doom can be turned into a memorable event
  and escape worthy of retelling.
  
 
  If monsters made it in with such alacrity, did also any of the magic? How many
  spells - or items, perhaps - were inspired by (or lifted from!) Anderson?
  Protection from Evil, in reference to the holy circles drawn to protect
  the party while it sleeps?
I'm not sure.
  I am less versed in the spell lists than I am in the monsters: but an avid
  reader might be able to find them where I did not - and maybe even tell me
  about it!
The Expectation of a Player
Lastly - to speak to our protagonist: Holger.
  Holger is a Danish-American engineer who returns to Denmark to subvert and
  fight Nazi occupiers during the World War II period. During that time -
  attempting to smuggle a person of interest out of the Reich's clutches, Holger
  is wounded on the beach: at which point he falls out of time and into an
  alternative Earth: wherein he goes on to meet fae and mythic creatures while
  questing about, seeking a way home. This sort of Isekai experience may sound
  familiar, reflected in the
  Dungeons & Dragons animated series
  as well as in the abandoned
  live action movie script
  Gygax had been working to produce while in Hollywood in the early 80s: but
  that's another topic for another time. To instead continue on the subject of
  the player, and what the original authors' expectations were of the player,
  one may consider three distinct moments in
  Three Hearts and Three Lions - note, spoilers to follow, but I will try
  to hide them in a details panel: if the panel fails, skip ahead to the
  paragraph beginning in bold with "as such."
  The Thermodynamics of Dragons
  
    Freshly betrayed by the Elf Lord, Alfric, Holger and his companions - the
    dwarf and the swan maiden - are accosted by a dragon: a servant of Chaos
    sent at the behest of Alfric or his ally, Morgan la Fey, to do away with
    them before Holger can fulfill his purpose: known to Morgan, but unknown to
    him or his allies. Hope is dim, the party convinced their adventure is at an
    end... but then this happens:
  
  
  
In this moment of hazard, Holger thinks quickly and acts
  unconventionally, saving the day with an out of the box approach. Holger knows
  about boiler explosions because - based on his background as an engineer and
  the timeframe in which this was said to take place, boiler explosions would
  have been something he - as a person - understood. His colleagues have no way
  of understanding what's going on - as boilers and the steam engines they power
  have not been invented in their timeline.
  
  
    In the context of the narrative - this makes sense for Holger to know.
    
  
  But would, in the context of the game, a player character?
 
  Cursed Gold
  
    Further along their way, the party rests - out of the Fey lands - but the
    Elf Lord Alfric has not yet given up - sending a Hill Giant to stalk them:
    waiting for a moment of "sinful thought" to annul the protection of holy
    symbols that they create or bear. When that moment comes - the Giant
    attacks: however, the party - recalling the bane that fey-folk have to the
    light - seek to entertain it with riddles, keeping it occupied until the
    dawn turns it to stone.
  
  
    In a series of silly half-riddles that result in the Giant furiously
    attacking Holger, the plan succeeds and the Giant, faced with the piercing
    rays of the sun, is turned to stone: leaving a treasure pile behind in his
    sack. Though wary of words of a curse on Giant's gold won in such a manner,
    the party begins to loot the target... until this happens: 
  
  
  
  
  
    Holger, as such, rescues the party from certain death - radiation poisoning!
    Though something that was new at the time of Holger's adventure in the
    1940s, a pre-atomic era (and a phenomenon barely understood even when the
    book was originally released in 1961), again - Holger uses his out-of-world
    (out-of-character?) knowledge to protect his companions: allowing their
    journey to continue.
  
 
  Magnesium Will Burn in Water
  
    Later - nearing the end of their quest - the band crosses by a loch at the
    base of a cave through which they must travel to reach the resting place of
    the legendary sword, Cortana. Cross, Holger separates himself from the party
    and - making himself unsavory with some equally cross thoughts - opens
    himself to attack, where he is kidnapped by a Nixie. How will he escape?
    Wits - and recalling a piece of equipment stolen from the Elf Lord, Alfric:
  
  
  
    The dagger - stolen earlier in the adventure and labeled "The Dagger of
    Burning" - Holger speculates to be made of magnesium based on its appearance
    and alleged inflammability. With it - and under the assumption that the
    ultraviolet radiation is what offends the creatures of Chaos, he puts it to
    use accordingly:
  
  
  
    Escaping certain peril - again, by his wits: wits tempered by the
    perspective of a modern man, a 20th century engineer, rather than a fantasy
    paladin. Arguably - in this case - it's the act of figuring out the
    operation of a limited use magic item: but still - figuring out the use of
    the dagger entailed knowledge of chemistry which Holger would have, but his
    allies - native to the setting - would not.
  
 
  As such, we find ourselves in a conundrum. In the context of the story,
  it makes sense that Holger will be able to use his life experience - his
  engineering background - to evade these hazards or to come up with creative
  solutions. However, in the context of the game - this brings to light
  an interesting concept: what Holger did, in each of these three cases, a
  modern Dungeon Master would cry foul as "Metagaming!" 
  What does this imply of the behaviors and expectations of players over player
  characters?
  What does this mean for the application of player skill and the understanding
  of what constitutes cheating - doing something your character would not: a
  problem which is identified in the 1e Dungeon Master's Guide - compared
  to what would be instead lauded a creative solution or application of talented
  imagination? 
  A lesson - to be sure - for a new OSR player or aspiring OSR referee: in that
  it indicates the solution may not be direct confrontation, that the solution
  may lie in a creative alternative which nullifies the problem to begin with -
  but also a point of pondering for the experienced ref: at what point does
  player skill end and metagaming or cheating begin?
  In 1974, perhaps, a lot farther along the scale, if Appendix N is to be
  believed.
And Them's My Two Coppers
  Thank you for reading! I hope, between the ten minutes prior to this line
  coming to your feed and now, I've provided some context and impressed upon
  you, dear reader, the significance of Three Hearts and Three Lions to
  the development and implementation of the original D&D game and the OSR
  experience - and provided a spur to the uninitiated to consume it, have you
  not already.
Stay blessed, all - and delve on!
Open license and public domain artwork retrieved from Pixabay and OldBookIllustrations.com and adapted for thematic use. Attribution in alt text. 
Three Hearts and Three Lions, written by and copyright to Poul Anderson, 1961. Images used herein taken from the Open Road Integrated Media edition, 2018 - with cover art by Jason Gabbert - all rights reserved by the publisher.
Image of Poul Anderson taken from Trader to the Stars, 1964, property of Knopf-Doubleday.
Dungeons & Dragons: Book II - Monsters & Treasure, written by Gary Gygax and Dave Arneson, as well as all content therein is copyright and trademark of Wizards of the Coast LLC, 2013.
Clerics Wear Ringmail claims no ownership of and defers all rights to the respective authors and owners of the properties referenced above. Any content or imagery used herein is used under US Code Title 17 Section 107: illustrative to and in service of criticism and commentary.