Feeling Good by Dr. David Burns
Amazon.com: Feeling Good: The New Mood Therapy Revised and Updated: Books: David D. Burns
It was my introduction to Cognitive Behavioral Therapy ( CBT ). I have yet to be happy with any one book on this subject as an introduction, but this is the one that got me into the subject and gave me my initial understanding.
CBT is based on the idea that our thoughts cause our feelings and influence our behavior as well as that all three of these things influence each other. CBT focuses on weeding out irrational thoughts, replacing them with positive, but realistic, rational thoughts and reenforcing those thoughts with behavioral modification.
The basic understanding I got from this book as well as better exercises and refinements i read in other books on the subject have made a big dent in a positive direction for me over the years.