Cliché, but "Awaken the Giant Within" by Tony Robbins.

It wasn't the book itself that changed my life, it was the exposure to all of the underlying ideas & philosophies that then significantly changed my life. I know many people can't stand the guy, but his book was really powerful for me at one point in my life. I got my shit together and started to understand the origins of many of the messages he promotes.

I like to think that there's a paradox of book reading in general in that you never remember what you read, but you change because of it.