So, you're a sci-fi fan?

Valentine Michael Smith, a human born and raised on Mars, arrives on Earth stunning Western culture...and judging us as a human who has never been exposed to any of our societies.

(I highly recommend all of his books. Whether you agree with his politics or not, he'll make you think.)

*edit* As suggested by molo1134, definitely read the unabridged version.

Niven's hugely influential 1970 novel of an outer space expedition to a mysterious object – a vast artificial world in the shape of a ring – that goes horribly wrong.

Clarke questions the survival of mankind in this science-fiction tale about Overlords from outer space who dominate the world.

While on a mission to the planet Gethen — a world whose inhabitants can change their gender — earthling Genly Ai is sent by leaders of the nation of Orgoreyn to a concentration camp. The exiled prime minister of the nation of Karhide tries to rescue him.

Dick's trippy novel tells of sophisticated off-world androids who turn against their creators, slip back to a post-apocalyptic Earth, and must be hunted down by bounty hunter Rick Deckard. The book inspired — albeit very loosely — the 1982 Ridley Scott film Blade Runner.

A band of psychologists, under the leadership of psychohistorian Hari Seldon, plant a colony to encourage art, science, and technology in the declining Galactic Empire and to preserve the accumulated knowledge of humankind.

Follows the adventures of Paul Atreides, the son of a betrayed duke given up for dead on a treacherous desert planet and adopted by its fierce, nomadic people, who help him unravel his most unexpected destiny.

Young Andrew "Ender" Wiggin, bred to be a genius, is drafted to Battle School where he trains to lead the century-long fight against the alien Buggers.

(Way, WAY, better than that thing that passed as a movie.)

Seven pilgrims undertake a voyage to the world of Hyperion — dominated by a fearsome and mysterious creature called the Shrike — where they hope to learn the secret that will save humanity.

In the first, hilarious volume of Adams' Hitchhiker's series, reluctant galactic traveler Arthur Dent gets swept up in some literally Earth-shattering events involving aliens, sperm whales, a depressed robot, mice who are more than they seem, and some really, really bad poetry.

(Also way, WAY, better than the movie.)

*edit* Late addition! Of all those suggested in the comments this is the one that made me say, "Oh crap! I forgot that one!" So, by popular demand, despite my senility, here it is.

Gibson's groundbreaking debut novel follows Case, a burned-out computer whiz, who is asked to steal a security code that is locked in the most heavily guarded databank in the solar system. A seminal work in the genre that would come to be known as cyberpunk.