Foundation and Empire (Foundation Novels): Isaac Asimov - Amazing stuff. Not as out-of-the-gate amazing as the book before, but it's the same story and thus must be read.
Isaac Asimov changed our perception of robots forever when he formulated the laws governing their behavior. In I, Robot, Asimov chronicles the development of the robot through a series of interlinked stories .
The Eye of the World (The Wheel of Time, Book (Mass Market Paperback)By Robert Jordan
Heinlein can be a bit over-the-top, but this book had a profound effect on me. And a very fun read.
Speaker for the Dead
Speaker for the Dead (Ender Quintet Scott Orson Card -"Ender Wiggin, the young military genius, discovers that a second alien war is inevitable and that he must dismiss his fears to make peace with humanity's strange new brothers.
1984 by George Orwell. Another very eerie dystopian novel that in many ways mirrors the direction in which our society is headed.