The Catcher in the Rye: forced my perspective to grow up.
The Sun Also Rises: the perfect book. This book taught me more about my grandfather's generation than anything else in my life.
Dune: While I still believed in god when I first read this book, it more than anything else I think contributed to me questioning religion's roll in my life. The uncertainty of the Golden Path just seemed, at the time, so relevant to my own pious and dogmatic religious path that I couldn't ignore it.
Ender's Game: This book taught me that children can be just as exceptional as adults, and communicated that message in a wonderfully entertaining way. I can't remember how old I was when I read this, but I remember the feeling of limitations being lifted off me.