As Kelly mentioned, it's hard to think of ten that I can't live without...but i'll do my best.
1. "Ender's Game" by Orson Scott Card
2. "Protector of the Small" quartet by Tamora Pierce
3. "A Swiftly Tilting Planet" by Madeliene L'Engle
4. "Knowing the Heart of God" by George MacDonald
5-7. Harry Potter #s 3, 4, and 6
8. "Touch Not the Cat" by Mary Stewart (or really any of her older books, other than 'thunder on the right')
9. "The Keeping Days" series by Norma Johnston
10. "Linnets and Valerians" by Elizabeth Goudge
I could've kept going, but really, all of these books have changed me in some way or another...of course, I think most books change me, to be honest. But definitely these ten (and the books that go with them) have helped me think differently, have opened my eyes to other worlds, other ideas; have enhanced my creativity and made me ponder life.