My picks, not mentioned in this thread so far:
Jack Vance's Demon Princes please!
Zelazny's Amber. (Not to be confused with the Chateau D'Amberville/Castle Amber material published way back by T$R.)
Julian May's Pliocene Exile.
Commentary on items already mentioned:
Dune: Oh Yes. Puleeeze someone do this.
Donaldson's Land: hmmm...it's a great literary setting, but... remember when you were a kid and you built a cool model car or plane, then stuffed it full of firecrackers for the sheer pleasure of watching it blow up?
That's my picture of Steve Donaldson writing about his Land.
He built this acheingly beautiful world and drops Covenant Schmuckface in there to "save" it by stumbling around whining while Lord Foul turns the place into a nightmare for six huge books' worth of abject misery.
The characters and the setting, and Donaldson's formidable writing skills made us want to stick around and read thousands of pages.
The whole makes for a great ironic literary comment on iconic fantasy, but as a D&D setting, wouldn't a typical party of PCs just take Covenant's ring away, fly to Kiril Threndor or Reject Home (er, Ridjeck Thome) and whup Foul's butt?
"Hey Regdar, some looney old man was yelling 'be true" at me so I cracked him with my Bastard Sword - whoa, where'd ya get that sick green rock?"
Greek Golden Age - great idea, as long as we don't get Herk-n-Xena, the RPG. I really like this idea and would buy it if done right.
Asimov's Foundation: Great, great books, but as Asimov himself said, there is absolutely no action (i.e. combat) in any of the books - so an RPG that's true to the source material would be pure roleplay, better suited maybe for a ruleset like The Window rather than D&D?
