A lot of classic literature is free for the kindle.
Dracula, Frankenstein, Paradise Lost, The Divine Comedy, The Origin of Species, Works of Edgar Allen Poe, Beowulf, The Canterbury Tales, Alice's Adventures in Wonderland, Through the Looking Glass, Faust, War of the Worlds, etc.
A few cheap ones: Brave New World (2.99), Complete Works of H.P. Lovecraft (0.99)
It really depends on what you like. If I had to choose my favorite of these, I would say Brave New World (my favorite book of all time). It's a futuristic dystopian novel, and hell I believe it's far more likely to come to pass than 1984.