So many choices, but I pick
As A Man Thinketh by James Allen. One of the earliest explanations of the mind's effect on the person. Can be read in a few hours.
As I read, I like to highlight the best parts. I'm in the process of collecting all the best quotations I've run across in my reading. Check out my collection at:
Quotation Collection Collecting the most inspiring, intriguing, and amusing words ever said..
Lots of positive, motivational, and inspirational nuggets there.