Stranger in a Strange Land by Robert A. Heinlein, 1961.
The "stranger" is a young man named Valentine Michael Smith (Mike), and the "strange land" is Earth. Mike was raised on Mars by Martians, and is brought back to Earth by a visiting human ship. Upon reaching Earth, Mike is immediately hit by culture shock. The Martians are infinitely wise and powerful, and Mike finds Earth to be anything but with its barbaric customs and backwards logic. The story is about Mike's attempt to adapt to Earth, and more importantly, the Earth's struggle to cope with the superior way of life that Mike brings.
While Stranger in a Strange Land is ostensibly Mike's story, in reality it is a vehicle for Heinlein's more controversial ideas. Most of Heinlein's novels push his political and sociological beliefs (barring those written for children), but none to the extent that Stranger in a Strange Land does. Unfortunately, this causes the book's plot to suffer, mashed together in a way more fit for convenience than artistry or entertainment.
Despite the limp plot, the shocking assertions that Heinlein makes should keep you absorbed (or possibly just offended, depending on your constitution). Heinlein breaks the rules through the actions of Mike and the voice of Jubal Harshaw, Mike's human mentor and father figure and Heinlein's mouthpiece. Between the two nothing is sacred - free love, group marriage, cannibalism, cultism and more are pushed by the protagonists.
It's debatable which of the ideas Heinlein truly held and which were there to provoke the reader. Group marriage for instance is practiced among clans in his novel The Moon Is a Harsh Mistress. Beyond pushing his own agenda, it was clearly Heinlein's aim to force the reader to question his assumptions and lifestyle, and in that at the least Heinlein succeeds marvelously.
Stranger in a Strange Land is a book you'll either love or hate. It fueled and inspired the hippy movement, inscribed new terms in the geek lexicon, and has appalled countless conservative readers. If you're looking for something different, you've found it.
