I haven't read it for years since I was quite young but Sophies world by Jostein Gaarder. I cant remember too much of it but it changed the way I thought as I was growing up made me question things a little bit more. It may not be the best book to a lot of people (will have to read it again to see what I think now) but it has had a profound effect on my life.
I have read lots of books cover to cover in a day and enjoyed plenty however none have had quite such an effect on me.