I've read this book, and one of the main subjects the book reinforces is the concept of forgiveness, of letting go of resentment and ill feelings.
After I read this book, I did do some of the homework the book recommends and I have to say, I think it had a very positive affect on my health and my life. Specifically my mental health.
It helped me to realise I have been carrying around bad feelings of things that happened years and years ago, Things that in many ways I had not realised I had carried. Releasing me of these feelings allowed me to let new feelings in. In a way I felt released to be happy again.
I do think that in order to get something from this book, and many like them you have to believe in the message they give you. However, I will add, I did not start my homework fully believing it would work, only simply hoping it would, but knowing it wouldn't cause any harm.