What I found most valuable in the book is Mitchell talking through advanced stages of Nei Gong training that he himself has experienced, such as transmuting qi to shen. Plus he gives clear explanations of earlier stages and is specific about what to do and signs to look for (though I wonder if people vary and some are bound to experience a different sign here or there). In no other book have I found these aspects so clearly laid out, and that's what makes this book quite valuable to me. He's a Westerner and plainly knows how to write to the linear-thinking Western reader. Though at times I felt he took that a bit too far. Unlike other reviewers here, I didn't appreciate as much the first half of the book, which goes over Daoist theory as it pertains to the body, qi, five elements, etc. Much of it is critical foundational info, but some of it was extraneous (e.g. the five elements), and I cringe a bit inside when these theories - and they are indeed theories - are presented as fact rather than interpretations of a body of experience. Yes, qi sensations are real and can be cultivated, but talking about things like negative and positive vibration waves in a manner that would probably run crosswise with physics is, I feel, trying to be too concrete with something that is ultimately unprovable, highly subjective, and interpreted differently in other cultures. To paraphrase the Tao Te Ching, "The Way that can be categorized neatly is not the true Way." So while I admire the breadth of what Mitchell has assimilated and presented (particularly at such a young age), I take some of it with a grain of salt. And that's fine, really not a big deal. I appreciated the few times in the book when Mitchell mentioned aspects of practice that he openly said he had not experienced. That sort of honesty is important in a book that is like a map charting unknown territory for followers.
I can imagine people who are new to Nei Gong getting very excited after reading this book, and being eager to start practicing. I would add two caveats. First, much of Nei Gong advancement is done through meditation, something I don't think Mitchell emphasized enough. The book has a lot of great theory, but is rightly loose on practice, since there are various movement exercises that could be used in Nei Gong. However, I can't imagine one advancing far without a strong foundation in meditation, and more space could be spent in the book on his views about meditation techniques, experiences, pitfalls, etc. Secondly, people need a qualified teacher to put all this theory into effective practice. I've never heard of anyone advancing far without roadblocks requiring a teacher's help. Mitchell advises people to seek out teachers for this or that aspect, but really everyone needs a teacher, often more than one. Even the Taoist classics state that, specifying that books are not enough.
Overall though, a very impressive book. Mitchell has outlined a process and levels of achievement that takes many dedicated practitioners 10 or more years, which makes this a great reference as we work our paths.