Some people may be skeptical about Bruce Lee as a martial artist, bodybuilder, fitness enthusiast, etc. But in reality one need not be a fan to understand the brilliance to his methods. The book always emphasizes that Bruce was all about changing things, looking and trying all aspects, then deciding what stays and what is unnecessary. Like chiseling away the pieces of stone to carve a statue.
After having done extensive research into bodybuilding, fitness, martial arts, Asian Medicine, Eastern Philosophy, exercise physiology, and dietetics I can honestly say that this book is very advanced. What I mean by this is that one has to do his/her own research to decide about the methods outlined. Indeed some of the research is outdated. For example the couplings of incomplete proteins and complete is slightly flawed in logic. However, you must not forget these methods were employed way before large advances were made in the way of sports nutrition and supplementation.
In the training aspect any normal human would indeed be overtrained. But the body is a magnificent creation and can adapt to anything that it encounters. Shaolin Monks, Ultra Runners, Olympic Gymnasts... they all do things that would overtrain any of us normal people. But they are not overtrained. Indeed Bruce had trained long and hard to become as resilient as such.
As someone who has done a little Personal Training I have to say that his development when he started lifting weights showed his ability to adapt. His body became very muscular very quickly. However, for those who think steroids were his thing... they were not. Not only did he believe in the natural way he also believed that a bulky body will decrease flexibilty and overall speed and endurance. So he actually lost muscle for the sake of being a better martial artist. At this time the only available steroids were things such as testosterone and Dianabol. Bulking agents. (Yes, some steroids build mass and others build power. Some even build endurance by producing erythropoetin but they weren't invented until the mid 70s. The most famous came into vogue in the 80s.) The pills he was seen popping so often were often amino acid pills.
My overall advice about this book? Get some training under your belt whether it's in body building, endurance running, martial arts, etc. Then read this book. It will bring you to the starting point to try and take your body to the next level. Anyone can be an armchair quarterback, but to get out there and try is much more practical.
Keep using and changing the things that challenge you and make a positive difference. Take away the things that do nothing or take away from your goals. If you're a body builder maybe running two miles a day every morning will be detrimental (Although for one who lives in Texas it actually helps keep him in the right shape to do his intense powerlifting/body building hybrid training). For an endurance runner perhaps having well conditioned forearms that enhance the hooking techniques of Wing Chun isn't necessary for running a marathon better. But for a martial artist perhaps a more conditioned midsection to enhance kicking and punching power might be of benefit to you. Or the added cardiovascular endurance to fight for 5 rounds instead of three.
As for the nutrition... remember. The only part that you have to remember is to eat foods that benefit you towards your goal and not eat foods that don't. Moderation is key as well. (A few cups of tea a day can make a LARGE difference)
As a personal experience... I used this book to not only increase the power of my midsection, but also to help lower my bodyfat, decrease my 2 mile time, strengthen my grappling ability, enhance my muscle mass, increase my energy, and also to reinstill that feeling of wanting to experiment. To find what works and what doesn't. The book is the gateway and not the means to an end. After applying the priniciples that suit you... it is up to you to keep learning new ways to keep improving.
If anyone is curious... I am a martial artist, body builder, dietician, Buddhist (born into the temple itself), scientist, and a sports nutritionist. And yes... I practice what I believe is necessary for improvement. Including the practice of taking a day off to eat anything, not train, not think, and just enjoy time with the people who matter in my life.