Some ideas, many of which I'm sure you have read (and if not, why not?):
- Edgar Rice Burrough's John Carter stories. They may be very silly, they may have a main character who is practically of Superman-level ability, they may be of the most throwaway quality (you'll forget 95% of the plot the instant you finish reading), but they are full of brio, imagination and atmosphere.
- The Suprising Adventures of Baron Munchausen, by Rudolf Raspe. A collection of the tall tales and far-fetched stories of a bragging Flashman-esque minor German nobleman who died in 1797. Amongst many other things he keeps bees for a Sultan in Turkey, throws a hatchet that flies all the way to the moon, shoots down a hot air balloon, turns a wolf inside-out, and slips from one side of the world to the other by falling into a volcano.
- The Gods of Pegana and Time and the Gods by Baron Dunsany. Twin works of sheer fantastical genius by a man cited by Lovecraft, Tolkien, Robert E. Howard, Moorcock, Gene Wolfe, Jorge Luis Borges AND Clark Ashton Smith as a strong influence. Was there ever a writer with as strong a D&D-inspiring credibility as that?
- Really anything published by Richard Hakluyt, as an important reminder of how big the world really is, and how renaissance or pseudo-renaissance societies were so very different and yet oddly similar to our own. His Principal Navigations, Voyages, Traffiques and Discoveries of the English Nation, composed almost entirely of eyewitness accounts, is hard to read but full of inspiration.
- Kubla Khan, from Coleridge's Ancient Mariner and Selected Poems, by Samuel Taylor Coleridge. Do you get more D&D than "Where Alph, the sacred river, ran/Through caverns measureless to man/Down to a sunless sea"?
- George MacDonald's Phantastes. I always preferred this to The Princess and the Goblin, which is really a book for kids. Phantastes is something else altogether. The scene in which the main character encounters his own shadow has stuck with me forever.
- Frankenstein, natch.
- Voltaire's Candide, which sort of reminds me of a D&D campaign committed to paper: a constant stream of improbable events, brushes with death, horrible occurrences, and bizarre absurdities. It maybe says something about D&D that it often feels like a satirical comedy from the 18th century. But that something is good.