For anyone interested in a very technicial analysis of the Tai Chi, as origionally taught by Yang Chengfu, I'd like to recommend 'Mastering Yang Style Taijiquan' by Fu Zhongwen, published by North Atlantic Books.
This excellent text can provide a template to determine how far your version of Yang may have changed from that fo Yang Chengfu.
Most impressive is the detailed description of the moving points of concentration (jindian) in the Grasp Sparrow's Tail sequence of the form. For the Yang stylist, this is a real find interms of the detail with this system.
This book is NOT the answer for somebody who wants a general overview of Yang Tai Chi Chaun. It is meant for the system specialist.
It also follows the guidelines of a really goot tai chi book. In that it may contain the compressed knowledge of 50 years, unfortunately you may have to put in the entire 50 years itself to fully realize what is there.
At the same time, due to the necessities of this being written in China when the martial application of tai chi was frowned on, there is only a little bit of Yang's martial applications touched with this work.
While that is sad, there is still more than enough material here to keep anybody who wants to work, busy for a very, very long time.
Bushi No Te Isshinryu www.funkydragon.com/bushi