This book by Damo Mitchell is a solid breakdown and guide to the foundations of this type of neigong practice. I haven't compared it to much other contemporary material published on such subjects in the past 10-15 years, but this guide surprised me with its clear language and attempts at thorough explanation of the context and material itself.
While I don't question Mitchell's experience or skillset and welcome his communication skills, I don't understand why there is no mention anywhere in the book of the lineage or school of what is being shared. As another reviewer mentioned, this book is a foundational primer on the practice of Quanzhen (Complete Reality) Daoist neigong, a school founded in the 12th century with teachings today most widely transmitted by the Longmen (Dragon Gate) lineage. Usually it's not a great sign when a teacher does not make any particular mention of their own principal teacher(s) and lineage. As of this review, Mitchell does not designate a formal lineage of transmission on his personal "about" page on the website for his Lotus Neigong organization. It says he is a full initiate in Longmen pai but he refrains from saying who his teachers are because they wish to avoid attention and are reclusive.
I don't think this material is illegitimate and I don't think Damo is dishonest, and given the trajectory of his training background, the author might have ended up learning in unique circumstances. Different lines of Longmen have their own idiosyncratic expressions of formality as well. My teacher is very traditionally formal: students begin neigong training by invitation only, and the material is not casually shared with non-students (nor is this stage the inner door process of full initiation). Other Longmen lines are more open and have a variety of techniques and practices which yield a healthy foundation of practice for lay students (see Wang Liping's books of translated talks, for examples distinct from either Damo Mitchell's or my aforementioned teacher's).
A distinct contrast to my example experience would be the various modern traditional martial arts teachers in China who welcome students from around the world to train with them for a couple of years, giving a license at the end so the overseas student can teach the style or some aspects of it back home (sometimes completely independently, sometimes as a "satellite school"). These kinds of licenses also vary in their significance (and are all very recent developments in an era in which these arts have become very different and very endangered); for obvious reasons many teachers might put together a "package" curriculum to make publicly accessible, while only a small number of select long-term students are privately trained in the full lineage transmission.
At the same time, hiding the source has historically in martial arts transmission to the west been a business calculation, keeping international students from going directly to the source of the material (and the inverse). I'm not saying that's what is going on here! But all these scenarios previously have and still do exist. There's also the usual questions with modern frameworks for classical arts: what lineage does a serious student in Mitchell's system trace themselves? Will they become an inner door student of Longmen as well? It's possible the school is just beginning, I'm not doubting these things, I'm just curious.
For those who are trying to begin this type of neigong training based upon the book material alone, or perhaps are trying to expand/supplement their background of internal/martial training, keep the practice in context as incomplete without the guidance of a teacher who can give feedback. For instance, certain portions of the author's overall curriculum for new students are particular to his own experiences and training background and martial arts styles, which differs subtly from some curriculums and martial frameworks. The way Mitchell does and teaches the structure when standing is articulating subtly the flavor of energy/power (jin) that eventually forms the root and movement of his martial arts practice. The power exertion and structure and rooting that he uses is similar to the kind used in Chen styles of Taijiquan, or perhaps some systems like Bajiquan or Pigguaquan, as opposed to Yang Taijiquan or Baguazhang. I mention this not because Mitchell is wrong in any way, but because the Chen paradigm of jin does not contain the Yang paradigm of jin within it, and vice versa. When certain famous gongfu teachers teach Yang style Taijiquan as just another additional curriculum of stuff next to their sets of Chen, Shaolin, White Crane, Mantis, Wing Chun, etc, they are not actually teaching Yang Taijiquan as their jin and structure is that of another art.
The last paragraph gets a bit beyond the scope of what is important here for most people though. And Damo's ethical and preferential guidelines are healthy perspectives and he recommends new students use proportional kinds of exercises and practices from their own background and system should that integrate better overall. But he offers his own honed curriculum if new students have not already developed a primary foundation, or perhaps are trying to start fresh. And I do think if you happen to be approaching this material from or alongside a Chen Taiji system doing stuff like Silk Reeling (or a Yang line in the approach of Damo's style), his preparatory supplementary exercises in his other books will mesh quite effectively into a full foundational curriculum.