In chronological order:
Martin of Redwall, by Brian Jacques
As a kid, I was into speculative fiction of various kinds, most particularly horror (Goosebumps series, some of the tamer Stephen King stuff), but the Redwall series was my first foray into traditional fantasy fiction, and I became immediately and totally hooked. From age 11 or so (or whenever I picked up Martin of Redwall) to age 18 or so, I rarely picked up a book (besides school books) that wasn't fantasy fiction, and that proclivity has directly informed my choice in friends and hobbies ever since.
Power Politics, by Arundhati Roy
I can't remember at all how I came across this book, but it was during my junior year of high school that I read it (the first time). Up to this point, I had been socially-minded and politically active, but in the narrow way of children and teens, seeing only those issues that touched my existence in some way. Roy's books, beginning with Power Politics, helped me to start thinking about international and global social issues, and also showed me that I could be truly passionate about problems that didn't in any way affect my day-to-day life. In a way, the book didn't change me as much as it might have, because reading it made me want to go into Political Science and I haven't taken a single Poli-Sci course, but the values it instilled in me I still use daily, in my career and in my social life. This book was also the first where I went through the text underlining, highlighting, and dog-earing pages - a practice which I've found immensely useful throughout my schooling.
The Sandman, by Neil Gaimain
The effect of the Sandman series was much less direct than the other two books listed here, but no less substantial. The mythology informs my peculiar type of humanistic faith. The central themes of responsibility, continuity, beauty in the bizarre, and most importantly, friendship, comprise some of the main themes that I try to ascribe to my life. It affected my life in a more superficial way, because its aesthetic beauty as a volume of pages contributed greatly to my desire to start collecting books. This may be also the only book (or series, in this case) which has actually changed me physically - I have a tattoo on my leg of Morpheus and Matthew, and a quote from the final book in the series, The Wake.