There's one thing Mortensen did really well: he knew his posing mistakes.
The first half of the book (at least, if not more) is dedicated to posing mistakes. Some of the mistakes are pretty blatant physical problems; bad posture, foreshortening and distortions, unfortunate wrinkles and uncomfortable positioning. Some are compositional problems: "cut" limbs, drapes and clothing creating odd illusory proportions and lighting issues.
The other big section on the book is about working with models. He broke down relationships into situations where the photographer is dominant, where the model is dominant and where there's a give-and-take between the photographer and the model. His actual working philosophy was a bit wacky, but there's some good information in there.
Those are the parts I really care about.
Mortensen was, not surprisingly, most interested in shooting female nudes, but he did cover clothing and how tiny changes could fix compositional problems.
There is also a big section on "costume elements." When Mortensen started, Western Costume was one of his clients, and he shot a lot of models using costumes from their warehouse. He hated nearly all of them, and switched to using simple elements that suggested the look he wanted. It's rather interesting.
In many ways, this book is a product of its times, and an interesting read for folks who are into 1930s Hollywood. It's also, as I mentioned, more about shooting women and female nudes than anything else. Still, some of the posing advice points out mistakes that wouldn't be "mistakes" when shooting men.
On the annoying side, the book design is a bit wonky. Figure positioning and numbering isn't always in a sensible linear order. Anything that's covered in one of Mortensen's other books gets only the minimal (if any) explanation in The Model. If you want to know what he meant by "Basic Lighting" you have to find a copy of his lighting book. If you want to know what he meant by "Projection Control" you have to find a copy of his darkroom book.
Still, if you can find a copy, it's worth reading, whether for the giggles over 1930's design and morality or for the useful advice.