Permanent, Pervasive, and Personal
The title is somewhat misleading - Seligman really spends more time discussing what he calls "explanatory styles." His thesis: you can change your explanatory style, and thus learn to be optimistic. He is careful to discuss the evolutionary advantages of pessimism, too, and shares his thoughts on when optimistic or pessimistic styles should be used.
This book places another brick in the foundation of my program. It fills in some of the holes for me. However, I don't think it was absolutely necessary that I read it, so I can't recommend it as a core text.