This book is great if you want to start as a personal trainer. If you've already established your business, it won't be of much use to you, except maybe for the ideas on how to promote it.
The book explains in detail all the necessary steps to launching a personal trainer business, from 'is this for you?' to how to get certified and what type of certifications you will need. It also offers useful tips on how to promote your business and, probably the most important, it gives you all the business forms you will need. This alone is worth several times the price of the book as having them done by an attorney can be very expensive.