In roughly chronological order:
The Velveteen Rabbit: freaked me out, but I love the story.
Wishbone books: first encounters with literary characters, plus Wishbone is awesome
The Arthur Cycle ~ T.H. White - I was really into Robin Hood and King Arthur for a while...
Nancy Drew books
Narnia series ~ C.S. Lewis
A Wrinkle in Time ~ Madeline L'Engle
Lord of the Rings and The Hobbit (in German) ~ J.R.R. Tolkien
The Count of Monte Cristo ~ Alexandre Dumas
Space Trilogy ~ CS Lewis
Watership Down ~ Richards Adams - sparked an interest in linguistics
The Works of William Shakespeare
The Brothers Karamazov ~ Fyodor Dostoevsky - Best. Novel. Ever.
Rosencrantz and Guildenstern are Dead ~ Tom Stoppard
V For Vendetta ~ Alan Moore
Sir Gawain and the Green Knight
1984 ~ George Orwell
Frankenstein ~ Mary Shelley
The Irresistable Revolution ~ Shane Claiborne
Howl and Other Poems ~ Allen Ginsberg
Great Expectations ~ Studied with Dr. Ruth Glancy, which made it that much more amazing
Jane Eyre ~ Charlotte Bronte
Leaving Home ~ David French
A Canticle for Leibowitz ~ Walter M. Miller Jr.
The Cripple and His Talismans ~ Anosh Irani
A Clockwork Orange ~ Anthony Burgess
Till We Have Faces ~ C.S. Lewis - probably my favourite of his fictional works
The Poetry of John Donne
Franny and Zooey ~ J.D. Salinger
Brave New World ~ Aldous Huxley