I can't pick just one.
Atlas Shrugged by Ayn Rand. I had never before been told to
think so many times and in so many ways. (And in so many pages.
)
If You Want to Write by Brenda Ueland. Turned me on to the idea that you can only be truly creative if you are free and honest.
The Selfish Gene by Richard Dawkins. I knew before I read this book what evolution was, but I had never been show how simple and elegant. This is what sparked the idea, which I am still passionate about, that everything is simple and beautiful if you peer in deep enough.