This is not so much a book about losing weight as a book about learning to take good care of yourself. Of course, the hope is that when you do that, you will naturally lose weight, and that is what the author apparently did.
It is structured as a workbook and you have tasks to do each day for 40 days. So I tried to follow the tasks, and all went well for a couple of weeks, although I did not notice any particular change in how I was viewing myself, my food or the world. Then I stopped, which I guess was my own decision, but it was because the tasks quite suddenly became a lot more difficult, both in terms of comfort zone and in terms of finding a place for them in my day.
So then I just read the rest of the book. I don’t think it had much effect, but that was predictable because I didn't follow the plan, and it was an interesting read.
The writer often quotes from A Course in Miracles. The religious side becomes much more evident towards the end of the book. I think you would have to be comfortable with that to follow through to the end.