I'm not sure the person who gave this two stars actually even read the book. I own this book, and also the companion about painting in oils and while there is not a lot of groundbreaking material, both books contain good solid instruction.
The beauty of Paul's work is that he brings focus from the larger shapes into the smaller details. The end results are portraits that have life and expression. Lots of books try to refine portraits to the point of photography, which makes them stiff and lifeless, and really...at that point why not just take a picture of the person?
The artist's job is to interpret what is before them and translate that onto the page. This book gives the artist the tools to do that - starting with material options and choices, bringing the artist through various features of the face, and finally into a half dozen or so detailed case studies.
Overall a good book for a beginning portraitist, and a handy reference guide for an experienced draughtsman.