This is the best book i've found so far discussing qigong theory (includes also a great chapter on Qigong history) , Dr. Yang has clearly a profound knowledge on the subject, not only on a theoretical level, since he pratices martial arts and qigong since he was a teenager. To pratice Qigong you must know the what, the why, and the how (the movements and postures are what is "viewable" but that is not the essence of qigong. This book intends to give you that kind of knowledge. The theory of qigong in it's deeper levels it's complex and a bit hard to understand at first, so, do not be surprised if you have to read a second time, or to look for other sources in order to be able to comprehend the theory.
Another positive point is that the author uses the modern Pinyin translation and includes the chinese characters of some words and expressions, which is great for chinese readers, or students of mandarin chinese. Being a physician he also presents a very interesting section with some scientifical hypotheses about Qi, bioelectromagnetic energy and the electromagnetic field in the human body. He presents this hipotheses in a skeptical aproach (not the anedoctical pseudo-scientific thinking we often see in people who "MUST" find a scientifical explanation for their beliefs or pratices. Dr. Yang's not ashamed of assuming his ignorance, and that's one of the best qualities someone can possess.
Keep in mind that this is a book to understand qigong theory, not for buy it and start practicing.
I only focused the positive aspects of this work, because in my opinion it's undoubtely a must have in every Qigong practicioner personal library.