I have read the book.
It was written by Cathy Stucker. She is an experienced mystery shopper, and she definetly is good at her stuff. When I was an editor, I read quite a few of her reports and that is part of what I base my opinion on.
The Mystery Shop Manual is sort of the basis of the Gold Certification (MSPA) workshop. There is a great deal of duplication and I would see no need for anyone to be exposed to both. I took the Gold Certification class from her, and she is knoweldgable and helpful. That is also part of what I base my opinion on. The book has a STRONG MSP influence (several of the MSPA board members and president at the time were involved in writing the book or giving input/content).
If you are a brand new shopper, or even someone who is thinking about getting into shopping and wants to know more before they take the plunge -I would recommend the book for the information on how shopping works, terminology, etc. As already mentioned - the links and resources can be outdated. I think they are updated once a year and obviously the Internet is often going to be fresher and more current.
If you are a current shopper that wants an MSP perspective that sometimes cannot be found on the forums (as they are shopper-centric) then the book might be worthwhile to you. But the actual information on how to shop is very basic and probably not anything new to you. If you are gold certified - then I would definitely never suggest the book - there would be nothing new I would suspect.