What I like about NASM's certification program for personal trainers is that they teach you to work on not just strength but flexibility, balance, nutrition, and the mindset of the client.
It's an integrated and whole body approach that allows all parts of the body to get fit before adding lots of pounds to their resistance training. It hasn't only helped me as a personal trainer advance through the ranks of my club, but also has given me the tools to really make some incredible changes in some of my clients (the willing one's anyways!).
If there were one criticism for this particular textbook, and really the certification in general, it would be the lack of emphasis on anything to do with selling yourself to clients. I found it especially difficult when I first started training in a club environment because I had never had a sales job, nor ever really wanted one.
I think a good addition to this textbook would be a book or program designed to help trainers (like myself) who have a true passion for health and fitness but not a very good handle on the sales and marketing part of it. I read a book called The Personal Training Sales Education Textbook: A simple and effective sales system designed specifically for the personal trainer in the health club. about six months into my career as a personal trainer, and have got to say it really opened my eyes to the fact that, yes this is sales but in order to be successful as a personal trainer I didn't have to give into the high pressured sales techniques that my first manager initially trained me on.
There are ways to communicate the value of personal training by linking the benefits of training (or more to the point, the positive feelings one experiences when someone reaches their goals) and to do it so that can convince themselves that training is something worth paying for. I guess you would call it "soft" or "indirect" selling.
I would say to anyone new to this business to first get the NASM book and certification for the reasons I outlined above, there really is no better, and then also find yourself a solid resource that teaches you how to sell personal training and more importantly, how to communicate the value of personal training and then position yourself and your services as the best solution to helping them reach their goals.