Heinlein - "Stranger In A Strange Land": I read this book when I was 20, and it completely changed the way I look at the world, and made me a thorough libertarian.
Ayn Rand - "The Fountainhead" & "Atlas Shrugged": I don't have a lot of use for Randroids, or even staunch objectivists (ie: I disagree with the idea that objectivity excludes the existence of a creator), but these books really changed me, and showed me the value of individualism, of personal accomplishment, personal excellence, and unwavering devotion to oneself. It reminds me to see the greatness in people, and not get too caught up in all of what we get wrong. I even named my dog Howie (for Howard Roark). He has a little bone-shaped name tag on his collar that says: "Howard Roark -- Dog."
George Orwell - "Animal Farm": Like a lot of people I was first exposed to this book in a high school assignment. I don't really think I need to go to any lengths to explain why I'd love this or 1984, since I think the subject material is pretty well known to most people.
Mark Z. Danielewski - "House of Leaves": Like the house in the story, this book is so much bigger on the inside than it is on the outside. There are any number of stories and themes to be found inside, and all of them with fascinating things to say about subjects as varied as soldiers coming home from wars, the relationship between a mother and her son, a dysfunctional marriage, etc. It's a story about people who are stalked by something in their past that they can't come to grips with. Or so I think. The book is so obtusely structured with layers upon layers that you could interpret it 10 different ways and never really be wrong. Beyond anything else, this book shattered my expectations for what fiction can be. It's even more moving when you compare it to the pseudo-soundtrack released by his sister Poe in the form of her album "Haunted." I loved the album by itself, but combined with this book, they blew me away.
Hunter S. Thompson - "Fear and Loathing in Las Vegas": The death of the American Dream, eminent quotability, journalistic genius, total debauchery, and the fantastic art of Ralph Steadman. This is really the book (and the movie, but the book first) that introduced me to psychedelics.
Just a short sampling of what would probably be a very long list.