Dawkins' The God Delusion -- not for the quality of the book itself (although it isn't bad) so much as it being my 'intro' to non-fiction with the exception of some Nietzsche when I was too young to really get a grasp on it. It opened up a whole new world for me. Science, history, philosophy....
Also Martin's A Song of Ice and Fire series since that really got me to dig into fantasy and into reading since he is bar none the best character builder I have seen thus far and also has an amazing grasp on the English language.
Goethe's The Sorrows of Young Werther is another one since that introduced me to his genius, although to be fair, I'd already been building a Goethe collection at that point...but, I digress. He's one of my favorite authors and what I'm sure will be a lifelong interest of mine.
EDIT: The Dhammapada translated by Ananda Maitreya also comes to mind since that was my introduction to Buddhism and while I am not a Buddhist, it kicked off my interest in that religion which has lead to a study of many texts and books on the subject and a developing interest in meditation.

