If one could only have one book on yoga, the new book, Classical Hatha Yoga by Swami Rajarshi Muni would likely be it. This is the only book on yoga the practitioner or teacher would ever need. It is probably the most comprehensive book ever written on the subject of yoga and, most important, it is written by a highly advanced Yogi, based on his personal Kundalini sadhana. Check it out on Amazon's Inside the Book Program, where you can actually see its contents as well as its summary. It can also be purchased at the Kripalu Book Store.