The Riddle-Master trilogy, which presents an intriguingly different kind of magic, but wanders a bit.
All the Lankhmar (Fafhrd & Grey Mouser) stories. Classics, highly recommended, get a little weird and pointlessly sexual towards the end.
Most of the Elric stories. Again, classics, and entertainingly decadent, but repetitive after the first few books.
All the Conan stories by Howard (People of Black Circle, Hour of Dragon). And again: classics, but a bit repetitive, and one has to be able to bludgeon through the occasional racism. Still, very glad I read them.
All the New Sun books (Shadow and Claw, Sword and Citadel). Very very good, very very weird. I recommend keeping an OED within reach while reading them.
The Chronicles of Amber. I read all ten books; the first is the best, and is highly recommended. They slowly drop off in quality as one goes on, with a bigger drop between the first five and second five. If you skip the second five, no one will blame you.
Little, Big. Lovely modern and postmodern fantasy. I want that house.
Tales of the Dying Earth. Very stylish, a bit meandering, but worth it for the set-pieces.
-Note that Dying Earth, Conan, Elric, and Lankhmar were all huge influences on D&D, second only to Tolkien, and are recommended to those looking for modern gaming's roots in literature.
-In the SF section, I've read most of them:
Rediscovery of Man. Cordwainer Smith is the poet of science fiction. Very different from most SF (and varies a lot within the one volume), and highly recommended.
Gateway. A definitive Precursors novel, and has an interesting structure, but not a radically distinct shining star.
I've read part of Lord of Light, no opinion yet.
Babel-17. An interesting novel in which the "science" in "science ficiton" is linguistics.
The Stars My Destination. The protagonist is a very bad man who eventually seeks redemption. Has some very nice bits, and some that seem kind of silly now.
Do Androids Dream. Surprisingly nutty-crunchy for the novel that inspired Blade Runner. Given the choice, I'd watch the movie again over reading the book again.
Cities in Flight. Blish thinks big, but I didn't find the Cities novels particularly gripping.
The Forever War. An anti-war classic, which uses time dilation as a symbol for post-traumatic stress disorder. Recommended.
Day of the Triffids. Read it once; it didn't do a lot for me.
Ringworld. Love it, though its flaws have become more apparent as I've gotten older. For fun, try reading it as a remake of The Wizard of Oz.
The Moon is a Harsh Mistress. Another favorite of mine, though the sexism grates a bit. (The main cast consists of a genius computer (usually male), a genius engineer (male), a genius professor (male), and a rabble-rouser (female). The last character's main role is to have things explained to her.)
Childhood's End. An absolute classic.
Canticle for Leibowitz. A bit patchy, but a very intriguing treatment of post-apocalyptic religion.
Man in the High Castle. Worth reading, but didn't do a lot for me, possibly because it's been so often imitated.
Left Hand of Darkness. I prefer The Dispossessed, but this is the definitive novel about androgyny.
Dune. The first novel is a must-read landmark classic; the film and TV versions are barely adequate to convey the intelligence in this book.
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-Note that Dying Earth, Conan, Elric, and Lankhmar were all huge influences on D&D, second only to Tolkien, and are recommended to those looking for modern gaming's roots in literature.
-In the SF section, I've read most of them: