Do Androids Dream of Electric Sheep, by Philip K. Dick.
Slaughterhouse Five, by Kurt Vonnegut.
Every Damn Sherlock Holmes book, by Sir Arthur Conan Doyle.
And goodness, has no one recommended the HHTTG-series yet?
If so;
The Hitchhikers Guide to the Galaxy, by Douglas Adams.
I reccomend A Connecticut Yankee in King Arthur's Court, by Mark Twain.
And if you read only one sci-fi book in your life, it should be Ilium by Dan Simmons. 3 completely different settings and storylines, based on the works of Homer and Shakespeare.
All The Kings Men by Robert Penn Warren
Heart of Darkness by Joseph Conrad
Cloud Atlas by David Mitchell
if you want to get crazy deep into literature.