"The first principle is that you must not fool yourself--and you are the easiest person to fool."--physicist Richard Feynman
Shermer starts with a discussion of two innate human tendencies that he has coined "patternicity," the bias toward seeing patterns whether they exist or not, and "agenticity," the tendency to assign the intention of an active agent to phenomena whether it exists or not. He then explores a number of other biases of the brain, such as an anchoring bias, a confirmation bias, and an authority bias and shows how they can cause us to misperceive the world.
Wisdom traditions from time immemorial have told us that we tend to be deluded in our perception of the world, but that there are practices available to help us see through those illusions. Shermer does a nice job at providing modern (and more-rigorous) explanations for the obstacles to our ability to see clearly and provides excellent tools for breaking through those obstacles.
For a taste of "The Believing Brain," go to: http://www.scientificamerican.com/article.cfm?id=the-believing-brain.