Thanks Mr Abersold.
Dianxue, your point is valid. I hadn't considered those two books you mentioned, which are quite old, and were published long before my time at YMAA. The fact that they say 'Volume 1' and 'Volume 2' is confusing. Each book has vital, inter-related aspects of basic Taijiquan training information, and so the books are considered a set. They are sold separately, and do not require one another to be a fully functional book, although it behooves the reader to do further reading, since the subject matter is quite varied, and Dr Yang's teaching is in-depth. These two books have been sold separately without complaint for many years, and they have become considered 'classics' in modern-day Taijiquan society.
You are not obligated to read the other book to understand the martial applications book. Dr. Yang has written over thirty books on various subjects, and he often recommends that we read another book dedicated to a subject in detail which is only briefly mentioned in the book at hand, such as the case of the Taiji Martial Applications book - which mentions that if you want to truly learn Taiji Theory deeply, which is recommended traditionally before you jump ahead in the training to studying martial fighting applications, then please first read the book about Taiji Theory. Some readers are interested only in Martial Applications. Some are interested in learning about Taiji Theory and how to develop Martial Power.
The two books mentioned were published in the 1980's, right after Dr Yang started his own publishing company, and it was unclear at that time if the books should be part of a "series" or not. The knowledge Dr Yang has to offer cannot fit into a single book, so a reader should not feel decieved.
The point is that Dr. Yang is on a mission to preserve and proliferate the traditional Chinese arts such as Taijiquan at a high level, and in doing so, he encourages people to train slowly, and in the traditional order, starting with learning Taiji Theory. It is his sincere hope that someone interested in Taijiquan martial applications will also learn the theory behind Taijiquan, which is traditionally taught first.
Sorry for the confusion. We appreciate your feedback. Thank you for writing.