I think this is a wonderful topic.
A Wrinkle in Time was a book that shaped me as well. I read a lot of books, particularly teen horror, but none of them were really, I don't know, "Good." I knew they were entertaining even then but that they lacked some smart quality I couldn't define. Then there was A Wrinkle in Time. It was full of adventures and complex characters. Characters like me. But it was also smart. Tragic and dark. Terrible and wonderful. To this day when I think of writing great adventures I think first of A Wrinkle in Time and how it made me feel when I read it. (Ditto for the Narnia series, but honestly A Wrinkle in Time is closer to my heart)
I was very, very young when I read this particular book, and I remember it scared me very badly the first time. But I checked it out at the library almost compulsively from then on. I think it propelled me into reading paranormal and horror as a teenager. Now I'm an adult writing teen paranormal/horror and I still think of this book as having a strange hold on the Scary Center of my brain. The book was called Talypo by Joanna C. Galdone.
On a similar subject, I devoured the Scary Stories to Tell in the Dark books. I'm starting to sense a pattern as to why I write what I write now!
There were lots of other books that touched me in some way too, The Polar Express and its beautiful, beautiful artwork (to this day I think more books should have artwork in them). Where the Red Fern Grows was the first novel to make me cry at the end (In class no less). All of Shel Silverstein's books deeply influenced my imagination. I started writing a book of poems because of him when I was in elementary school, with drawings, that I thought were hysterical. They mostly made fun of boys in my class, teachers, and my parents. My parents, by the way, thought they were a lot less hysterical than I did.