This is a great book. His other book, "Feeling Good: The New Mood Therapy" is also great.
The biggest idea I took away from both these books was the fact that people's moods and feelings are caused by their habitual thoughts. How you interpret your daily experience plays a big part in whether you are insecure and anxious or secure and feeling good.
Basically the ideas in these books allow you to start realizing that YOU are the one in control of your feelings. You aren't the victim of outside events because you always have the freedom to interpret those events however you want.
And the big challenge to get over is the habit of always choosing the most negative interpretation. Anxiety distorts your ability to see events objectively and accurately for what they are.