i have been studying yoga for years now. I was given this book by my father. It was around the house my entire life.
http://www.amazon.com/Yoga-Philosophy-Patanjali-Translation-Annotations/dp/0873957296/ref=sr_1_1?ie=UTF8&s=books&qid=1247397312&sr=1-1
this book is essentially just the sutras of patanjali with annotations on each sutra from another fairly ancient master and short suggestion from the swami that arranged the compilation of the volume.
It is difficult to to read, but if you can plug your way through it, you will find that "yoga" as it is is understood by 99% of the people out there is not yoga. Just unbalanced systems of exercise. The sutras of Patanjali are like a science book not an treatise on staying fit or finding bliss. Things like Hatha, Pilate, and the rest seem to take one specific part of the yogic system and expand upon it greatly to occlusion of the rest. Allthese other systems cropped up many hundreds of years after the original yogic sutras were written.
Liberation is the goal of yoga and that is attained through recognizing the self. There are maybe two paragraphs with breathing exercises and two with physical positions to hold ( and there are only four rather comfortable positions I might ad). the bulk of the book is about the tools of discovering yourself. The fourth Way is the only other book i have read that comes close to touching on the systems, order, frame of mind needed to discover yourself through the veil of the physical body and mind. First page of the book even gets you started by bringing concentration to 5 states of mind, the fifth state "arrested" is the state achieved when you have brought the other four states under your control. So we even have the same designation of four MAIN states of consciousness
Attaining samadhi (highest concentration) through yoga would be akin to reaching Ouspensky's "objective consciousness" and acquiring the higher mental an emotional functions.
there is a sutra that specifically details kriya yoga which means - yoga in action/movement which entails NOT withdrawing from society and becoming a "hermit on the mountain,"
I think in the end, of all the people doing yoga today, almost none are actually doing yoga. yoga really comes down to studying and dissecting your mind, something which must be done as you are living your life. Every moment of it. If you have withdrawn into solitude you have nothing to work with after a while, just holding positions and breathing do nothing other than settle your mind in preparation to work on it. they are not even the means to the end.