What do you know about Dungeons & Dragons? About where it came from and, specifically, what inspired it? Not the rules, but the fictional content of the game? If you follow the "OSR" movement or are simply an AD&D player since the first edition of the game, you might have a pretty good idea... The original Dungeon Masters Guide (1979) featured a slew of appendices from the verbose Gary Gygax, but one in particular—Appendix N—has consistently received the most attention.
Basically, said appendix is a list of fiction literature that inspired the game's content originally. (To quote Gygax, "From such sources, as well as any other imaginative writing or screenplay, you will be able to pluck kernels from which will grow the fruits of exciting campaigns") It's a useful list to reference when you're trying to convince someone that D&D is not purely some hardcore Tolkien-deification, as is common perception. There are a ton of authors presented there, and some of the biggest fictional characters from that list include Conan (created by Robert E. Howard), Elric (by Michael Moorcock), and Fafhrd & The Gray Mouser (by Fritz Leiber). A whole bunch of "sword and sorcery" novels populate the list, as well as some science fiction authors such as Leigh Brackett and Andre Norton.
I would argue that none of them capture the spirit of what D&D can and should be better than The Dying Earth by Jack Vance.
|This is the copy I have|
Perhaps you are even aware of one of the greatest villains of D&D lore, Vecna. A legendary lich, his disembodied eye and hand are legendary artifacts of the D&D world. In case you haven't worked it out yet, his name was simply a rearranged form of "Vance."
The point is, this author had some serious influence on the foundation of D&D. And yet, I see so little of this book manifest in modern-day D&D games. Unlike the mighty Conan, the tormented Elric, or the plucky Fafhrd and Gray Mouser, I dare say Vance's work has been cast aside out of the consciousness of most modern gamers.
Originally published in 1950, The Dying Earth is a series of loosely connected short stories depicting a fantasy world that is in fact Earth in its final years. The sun is read and bloated; magic is prevalent, as is ancient technology. There are six short stories, each named after its primary character. Perhaps the most directly related to D&D is "Mazirian the Magician," which firmly establishes the Vancian magic convention as the title character prepares a short list of spells, all that his brain can contain.
But if all you get that is game-worthy from The Dying Earth is the mechanics of magic, then you are not easily inspired. Take, for example, Chun the Unavoidable, a being who lives in ruins and snatches the eyes of anyone who steals from him, and then wears the eyes on a cloak. Or Ampridatvir, the city full of two sects of people who can't see each other because they're wearing two different colors of clothing. The universe which houses all these stories employs a fanciful sense of logic, which flies in the face of the ho-hum vanilla settings that you're used to in mainstream D&D.
The prose is a workout, to be sure, with quite poetic and fanciful language. It's not a style that suits those who don't like to take a hot, soaking bath in the nuance of the English language. Furthermore, the rest of the Dying Earth series focuses on Cugel, a largely unlikable antihero whose character made me put down the second book (Eyes of the Overworld) after just a few chapters. But I would highly recommend reading through the first book as an inspirational collection of shorter picaresques.
If you're interested in Jack Vance, Patrick Stuart did a thorough review of the Lyonesse trilogy (spoiler warning) here. I haven't read it yet, but it's on my shelf so I have no excuse. I can recommend the novella The Dragon Masters, although it's science fiction rather than fantasy.
Once you have read The Dying Earth, I would strongly recommend The Pastel City by M. John Harrison, the introduction to the Viriconium saga.