When I was, perhaps, 14, I found myself in Gimbels' book department, where I spent 75cents on two volumes of Edna St. Vincent Millet's poems, and a copy of Sinclair Lewis's Babbit.
Reading Millet convinced me that I wanted to be a writer. (Though I do not write poetry.) Lewis taught me to question.
Though, I suppose the book that MOST influenced my life was James Thurber's "Is Sex Necessary?". My parents were reading it when they started dating. It was one of the things that brought them together. Obviously, they decided it WAS necessary. The book is still in print. Read it! It's a howl!