Double King is OSR's Appendix N. Nothing is OSR's Appendix N.

I asked what literary/film materials people like to think of as representative of OSR play. (I can't find the G+ thread in which I asked.) Chris Wilson asked a similar question recently: "Let's say I'm completely new to OSR. I'm looking for good sources of inspiration to emulate, wether it be literature, film, theater, etc. What would be #1 on your list of recommended sources?" Here's a similar thread on Twitter.

If I had to choose just one written work that represents expected, standard OSR play to me, it would be Vance's Lyonesse trilogy. It's closer to actual gameplay in a typical fake medieval setting with magic in it than anything else I know of.

If I had to choose just one film, "Double King" is the only thing I know of that really answers the question in a way I'm satisfied with. Double King is basically a montage of OSR mechanics in action. And this is touching on the point I'd like to make.

In practice, Spirited Away is the biggest inspiration whenever I create something new for my game.

I have one objection to the idea of saying that some great literary works (including film) are relevant to OSR and some are not (note that I'm objecting to my own question. I love creating an Appendix N):

Old school roleplay is not about narrative genre. It's about design goals realized through mechanics and intent. I'll come back to this in a bit.

Baron Munchausen IS the real D&D.

Noisms (David McGrogan) likes John Carter stories, Baron Munchausen, Gulliver's Travels, Laputa, The Histories by Herodotus, David Copperfield, Egil's Saga, The Wind in the Willows, The Man Who Would Be King, William Blake's works, and Shakespeare's works as separate Appendix Ns. Most of these don't resemble the works that would typically be on the list, but they work.

Here's a Wild West Appendix N.

I'm not sure if every genre is OSR. However, most narrative genres I can think of are. Any of these would make a suitable Appendix N: Winnie the Pooh, World War 2, The War of the Roses, Stephen Universe, Phantastes, Alice in Wonderland, Crime and Punishment, A Tale of Two Cities, As I Lay Dying. It would be harder to make To the Lighthouse a primary source of D&D inspiration, but that'd probably be my favorite D&D game. Orlando, on the other hand, would be easy.

OSR is a set of design principles. It's a structure to put things inside of. It's almost a core set of rules, though they might be alterable without removing the OSR design. For me, OSR is summed up by the following:

• The GM must, in order of importance, simulate, infer, extrapolate, describe, and create with consistency. Combat is not the center of the game. Player characters who disagree will not live long.
• Players, when facing uncertainty, must roll in relation to an appropriate stat. They also must manage resources and apply cleverness to accomplish goals.

Double King represents the results of these principles applied across a long campaign by a very capable player. It's abstracted and makes "good play" look easier than it really is, but mostly represents it well.

P.S. Here are some free audiobooks from the original Appendix N. Here's a Goodreads list.

P.P.S. Gabor Lux said something in the comments that is well worth adding to my post, so I'm going to quote him:

[...] You can apply the mindset to many different sources, and you can draw inspiration from very different directions. I personally read very little genre fantasy anymore, and get my ideas from nonfiction, history, and the daily news.

I also think that there is a more narrow take on old-school gaming, and here, it pays to be more strict. Particularly for beginners to the playstyle, I would recommend to read at least part of the original Appendix N to "get" where Gygax and Co. were coming from. It is a good exercise not just because some of these books are very good, but because it is eye-opening to realise that there is a particular style of pulp fantasy where the strange bits of D&D suddenly make perfect sense. (While its aesthetics and assumed playstyle clash with either epic fantasy, or most of the modern fantasy tradition.) Once you have a deeper understanding of this style, it is very easy to branch out - but the fundamentals matter.

And this brings us to your main example, Lyonesse. That's an excellent recommendation, because while Lyonesse was written after the game's publication, it is very AD&D. It is a setting based on European legends and fairy tales that simultaneously feels very odd, even out of sync with its supposed roots - and that's very AD&D. Also, Vance often writes in a way that the situation his protagonists face are set up as game puzzles, allowing them to learn its mechanics and exploit them to their advantage. This is not simply the case in Lyonesse, but a mainstay of his fiction from Planet of Adventure (which would have been my pick for the #1 novel to read to understand old-school gaming) to The Demon Princes.

I agree. I think most OSR games would improve for players and GMs if everyone is familiar with Jack Vance. The Dying Earth and the Lyonesse trilogy are what I'd recommend to someone who wanted to read D&D but also wanted to read great literature.