Some days I feel like Wildbow and I are eerily similar in our mindset, as I can't say any book in particular changed my life, and I think I was always going to be a writer. Books in general, as a concept, are to my mind what water is to a fish. When someone asks me my "favourite" book I kind of go blank because I like them as a species and not as individuals.
However, there are a few stand-outs I suppose:
The Hobbit by J.R.R. Tolkien -- my dad had an illustrated version with pictures from the cartoon film, and I thought it was a children's book as a result, so I read it when I was 5 and grew up thinking it was for kids.
The Chronicles of Narnia by C.S. Lewis -- I figured out Aslan was Jesus and suddenly became aware of symbolism.
The Dark Tower series by Stephen King (particularly volume 4, Wizard and Glass) -- it's King's Lord of the Rings, and much better than his usual stuff.
Robert Heinlein -- I've read almost everything, but the standouts are Stranger in a Strange Land, Starship Troopers, The Number of the Beast, The Moon is a Harsh Mistress and Friday.
DragonLance Chronicles, by Margaret Weiss and Tracy Hickman
The Sparrow and Children of God by Mary Doria Russell
William Shakespeare - particularly Macbeth, King Lear, the Tempest, Richard 2 and 3, and the Taming of the Shrew.
Peace like a River by Leif Enger
A Prayer for Owen Meany by John Irving, and the Cider House Rules (though I like the movie better, oddly enough)