I have to admit, choosing just one of Ursula Le Guin’s books to suggest for the Book Club (Earthsea! The Dispossessed! some of her newer work!), but I kept coming back to The Left Hand of Darkness. Because it’s awesome.
(And I also love her because she translated one of my favorite speculative fiction books in Spanish, Angélica Gorodischer’s Kalpa imperial, to English.)