This is the book to refer to get a proper understanding of the philosophy of Yoga. I have gone through other books ( B. K. S. Iyengar, Georg Feuerstein, Swami Prabhavananda ) and some audio cassetes on the Yoga Sutras. This is by far the best book on the subject. The sutras are beautifully explained in layman terms with ample annotations. The sutras in samadhi pada and sadhana pada are covered extensively. Whenever there was a discrepency in the translation among different authors (such as sutras 18,19,33 of sadhana pada, which have been interpreted very differently) I found the translation and explanation in this book to be most logical, intuitive and complete. Swami Satchidanandas book on the Sutras is one of the best for digging into the background and foundations of the Sutras, and his commentary, unlike Iyengars and most other eastern commentaries, is clear, simple and easy to understand. Swami Satchidananda is clearly in touch with the truth that he is illuminating with this book, and its also clear from the powerful tone of authority with which he writes that he has reached the state of awaking which is the end purpose of the Sutras. It is rich in historical information and its tone is true to the purpose of the Sutras. I would recommend it for for all aspiring yogis.