Flow, by Mihaly Csikszentmihalyi. This book spells out the keys to a 'flow'mentality, at work and at play. Essential reading.
Think for Yourself!: An Essay on Cutting through the Babble, the Bias, and the Hype, by Steve Hindes. An essential book on critical thinking. It helped me to rethink my beliefs (or lack thereof).
The End of Overeating, by David A. Kessler, MD. By looking at the reasons behind current US obesity trends, Kessler is able to make solid recommendations for individuals trying to fight the overeating habit.