Elster opens his book by posing some questions which rational choice theory has had difficulty answering, such as:
Why do some gamblers bet on trends, and others on reversals?
Why do humiliating initiation rituals make group members more and not less loyal?
Why does switching prescription drugs from bottles to blister packs significantly reduce the incidence of suicide?
Why do stocks offer a much higher long-run return than bonds?
Why can a reputation for irrational behaviour improve your bargaining situation?
Rational choice theory -- which assumes that people objectively and unbiasedly pursue their goals -- has had little success in explaining these (and more) phenomena of the real world. Elster invokes a number of different fields, such as psychology, behavioural economics, neuroscience, biology, history, and political science in search of sensible answers.
Not only are Elster's answers intuitively appealing, but they also eschew the mind-bendingly complicated mathematics and implausible assumptions of rational choice. However, he doesn't entirely denigrate rational choice: we do, after all, want to act more-or-less rationally in life, and Elster provides a number of mechanisms by which we can attain greater rationality in our decision making.