This is a very good book in many ways, but it has one drawback that I think is very serious. Basically, the authors do not explain that the exercises they describe may lead to pain and frustration instead of increased well-being. They do point out, briefly, that if doing one of the exercises is uncomfortable, the reader should "feel free" to stop. This is not, however, nearly enough.
Let me explain.
The aim of the book is to guide people to increase the frequency and power of positive emotions in their lives--emotions like equanimity, compassion, gratitude and joy. (And, of course, to decrease the power of negative emotions like fear and hate.) There are a number of ways to do this, but the technique which the authors describe in the most detail is guided imagery. In guided imagery one imagines a situation that will trigger the desired emotion. Each time one creates these emotions, one strengthens their pathways in the brain/mind and thus makes oneself a happier/better person.
The problem is that when some people do this imagery they are unable to generate the intended feelings. Instead they feel disappointment and frustration at being unable to do what comes so easily (it seems) to other people. If the person has a history of failure at trying to improve her mood, and if the person has been told all her life to cheer up, look at the bright side, etc., than this can be quite painful, and, ultimately, psychologically harmful.
To see if these methods will work for you, try calling up some happy memory and see if it makes you feel happy. If it does, buy this book. There's a lot of good stuff here. If it doesn't, I recommend trying "The Mindful Way Through Depression". It has much of the same material but it is directed at people who have experienced long-term mental pain--not just depressives, but also people suffering from anxiety, chronic pain, and so forth. It is a tremendously good, useful, insightful book. (No, I have no connection with the book or its authors. I just think it's a great book.)