I have to second the "Spark" recommendation. It presents the very compelling research behind exercise's cognitive and mood benefits. I've read many of the articles and books on this topic and Spark still included new information for me.

After reading it I decided to finally get on a religious exercise schedule and I've been on it for two months. The improvements to my will-power, focus, and mood have been tremendous.

As a side note: one thing that has helped me keep up the 5 to 6 exercise sessions per week is reminding myself that I'm only exercising for cognitive and mood benefits. This helps me personally not get distracted by other factors like fat-loss, improving running times, etc. If I added other non-brain goals, then it'd be easy to get bummed out and discouraged if I didn't achieve them, so I keep my goals purely brain focused. If those other benefits happen as a result of the exercise, then great, but they are not why I'm doing it.