Often, the more influential books I've read are catalysts for epiphanies I have, and often aren't in the direction the authors "want" me to go. But a good book presents ideas, and the evaluation of those ideas can do a lot.
Age 18: Grendel by John Gardner (2nd read)
Age 21: Foundation by Isaac Asimov and The Decline and Fall of the Roman Empire by Gibbon (in conjunction)
Age 23: The Lore of Running by Tim Noakes
Age 24: The Idiot by Dostoevsky
There are plenty of books that I've read and really, really liked (e.g. Once a Runner, Brothers Karamazov) that just reinforced the way I already thought, but the books listed above changed me.