This book analyzes will, mainly problems connected with willpower, as a form of intertemporal bargaining between a current self that highly values immediate temptation and future selves who prefer that current choices be more far-sighted. He contrasts simple models of rational agents who exponentially discount future utility with his more sophisticated and complex model of people whose natural discount curve is hyperbolic. Hyperbolic discounting causes time-inconsistent preferences, resulting in problems such as addiction. Intertemporal bargains can generate rules which bundle rewards to produce behavior more closely approximating the more consistent exponential discount model.
He also discusses problems associated with habituation to rewards, and strategies that can be used to preserve an appetite for common rewards. For example, gambling might sometimes be rational if losing money that way restores an appetite for acquiring wealth.
Some interesting ideas mentioned are that timidity can be an addiction, and that pain involves some immediate short-lived reward (to draw attention) in addition to the more obvious negative effects.
For someone who already knows a fair amount about psychology, only small parts of the book will be surprising, but most parts will help you think a bit clearer about a broad range of problems.