That's the best book of Yang Style Taijiquan i've ever read. Much better than the others (including the book of Yang Chenfu, and Zhen Manqing and Yang Zhenduo's books). Its incredibily clear and detailed. But that's not a beginner's book, its for those advanced students who already knows the traditional yang long form.
Pretty much what you are looking for in a Yang family style book. It will not teach you Taijiquan, find a teacher for that. It will help you look upon what you are doing better and with a wider eye. It is very interesting, and if you are asking questions about the forms and stuff you are learning, this will answer them.
A definite must for any Yang family Taijiquan lookers ;) Very nice, Very much worth the money, yet it should be worth more, in my opinion.