It's all about your wonderful brain today. Here are two "brainy" books by David Sousa:
How the Brain Learns
How the Brain Learns Mathematics
Also, take a look at these exercise-the-brain books:
Building Mental Muscle by David Gamon et al
Get Your Brain in the Fast Lane by Michael Noir et al
Workout for a Balance Brain by Ken Russell and Philip Carter
Try AARP's puzzles/games to stimulate the brain.