If anyone's interested in the Aztecs or the Conquistadores then Aztec by Gary Jennings is a great read. It's an historical novel and I'm not usually a fan, but it really brings to life the Aztec culture.
Anything by Philip k dick, and A Scanner Darkly and Valis in particulaare worth a look, although as with all of Dick' stuff they can become a bit similar and many are just bizarre. His short stories are also worth a look, especially The Defenders, Paycheck and The Skull.
Finally, Evolution by Stephen Baxter.