[mattivc]

A great one that haven't already been mentioned is "Stranger in a Strange Land" by Robert Heinlein.

http://en.wikipedia.org/wiki/Stranger_in_a_Strange_Land

It's one of those books that just takes a while to digest after you are done reading it. I highly recommend it.

[partisan]

Very good book and the one that got me started on Heinlein. I probably own most of his hard sci-fi books now as a result.