A very worthwhile book is now available again from Plum Publications.
Wujishi Breathing Exercises is only about 100 pages but it details many of the intricacies of breathing both during static mediation and during dynamic martial arts. The book is a collection of explanations from the most illustrious Cai Song Fang. At least the English language version of 1986 went out of print but has, with much labor and a great deal of love, been resurrected by the fine folks at Plum. More about the book can be found at http://www.plumpub.com/sales/bagua/collbk_yiquan.htm. Also mentioned on the page are two books by Jan Diepersloot. Jan’s web page is http://www.warriorsofstillness.com. There’s a fair amount of material about Cai Song Fang in Qigong of the Center-Essence of Taijiquan which is volume I. Now out print and out of stock at Plum. The Tao of Yiquan – The Method of Awareness in the Martial Arts is volume 2 and it is, I believe, available. Volume 3 has a working title of Warriors Of Perception: Sensory-Motor Integration in the Internal Martial Arts. It is several years late in getting published. Send an encouraging note to Jan.