True, as some reviewers commented, some of the photos in this book are 'ugly' or, more accurately, aim to do something other than simply flatter the subject: this book will not help you take soft-focus portraits of your family and it has no section on pet portraiture. But I hope most people will find that quite refreshing.
It's a book for someone who wants to broaden their idea of what portraiture is about or a photographer looking for new inspiration. Although it contains hints, it's not a technical guide. Rather it introduces you to a variety of surprising stylistic themes and the history and theory behind them: e.g. portraits that slice the subject with the edge of the frame; out-of-focus subjects; 'voyeuristic' portraiture (through a keyhole, window, with a hidden camera etc.). Each theme is very engagingly and clearly explored and excellently illustrated with many examples from photographers both contemporary and historic. To push you even further, each chapter comes with a pretty difficult assignment. Again these are often quite unusual -- e.g. a self-portrait without your face or a long-exposure, light-painting portrait -- but with clear conceptual and practical pointers.
It teaches what are, perhaps, the sort of lessons you'd expect to learn if you were taking photography as part of a fine art course. In short, this is a book that will force you to think laterally about your portraits, to try something altogether new -- for me, that's exactly what I needed.