As a fashion design student, I was required to purchase this text several years ago. My drawing teacher insisted it was the industry standard, but I certainly don't think it's the best. While it provides good information about proportion and sketching the elongated fashion figure, the croquis in 9 Heads just aren't that attractive. They're often in very strange poses and the heads seem to be very large, while the necks and limbs are a bit thick, giving the illusion of a clunky figure rather than an elegant, slim one. While that's certainly easy to fix on your own, sometimes you just need a good croquis right away, which is why we buy books like this! It just doesn't deliver. There are very few back views, and the sections on drawing men and children are very limited, so don't buy it for that. Furthermore, the book is large and unwieldy, and it is impossible to make it lie flat, so it's very difficult to trace the croquis from it - something very important in a croquis book!
This book also showcases great fashion illustrations from various artists and designers interspersed throughout, which are inspiring, but can also be frustrating since they look NOTHING like the provided croquis and the book doesn't do a good enough job of instructing how to make your drawings look like that.
This book is great for helping with flats, though, especially in the "Encyclopedia of Details" section, which offers tons of examples. That's definitely the part I use most often. The sections on drawing hair, faces, hands, and other details are also pretty decent.
I recently acquired Bina Abling's "Fashion Sketchbook" and would definitely recommend that over 9 Heads if you can only afford one. The poses are much better, and it has many other techniques not found in 9 Heads, such as step-by-step instructions to render different fabrics realistically.