Ok, I’ve bought the book on Kindle and I’m most of the way through (it’s short and not particularly difficult reading).
Firstly, the book is 100%, totally focused on the classical guitar. The specifics of left and right techniques demonstrated simply do not apply to a steel string acoustic guitar or an electric guitar. As for how valuable the specific mechanics outlined are for the context of classical guitar, I am not qualified to say. However, they are very far from optimal for an acoustic guitar or an electric guitar.
There is good advice and some senitments I totally agree with. For example, I’ve long criticized the typical conception of “economy fo motion” to be overly naive and unhelpful, and I totally agree that large, effortless movements.
However, there’s a lot of nonsense too. There are statements which the author claims are based upon anatomical reasoning, but which either have no basis in anatomy which I can recognize, or which are totally at odds with our actual anatomy.
For example, he claims that the muscles which flex our MCP joints are larger and more powerful than the muscles which our PIP and DIP joints. This is categorically false. Our MCP joints are flexed primarily by our lumbricals, which are small, weak muscles intrinsic to the hand. Our PIP and DIP joints are flexed primarily by Flexor Digitorum Superficialis and Profundus (respectively), which are larger muscles in the forearm, and which are much, much more powerful than the lumbricals.
Moreover, when discussing elements of posture and positioning, there’s really very little acknowledgement of the degree of variation in individual anatomy. For example, there is very significant variation in human hip anatomy, and the “middle range” the author specifies for abduction and rotation is not universal.
I would honestly recommend that electric or steel-string acoustic guitar players just ignore the book entirely, and that classical players take everything stated with the knowledge that if something works, it may not be for the reasons implied, and hold it in mind that they may need to make allowances for their own specific anatomy.