I've been staring at a blank screen on and off for 3 hours trying to think of a book that would fit the bill, and I really can't. I can't think of any book that has actually changed the course of my life. So I'm going to go for one that I think has had a reasonably large impact on my life. It was quite possibly an influencing factor in me becoming a teacher, although not a conscious one. That is Anne of Green Gables by L.M. Montgomery. I loved Anne and wanted to be her, although I was probably more like Diana, though.
Honourable mention goes to The Alchemist by Paulo Coelho. Lovely book which may have encouraged me to travel to England.