A while back, I reviewed Ashtanga Yoga: Practice and Philosophy. It was a unique book that not only examined yoga postures in detail but also looked at the meaning and underlying purpose of those acts. Essentially, this resource served to give back the intention and purpose behind yoga. Although extremely detailed, I found the work fascinating.
Ashtanga Yoga: The Intermediate Series is the logical next step in understanding yoga. Again, the author looks at postures in detail, this time showing more intermediate positions. Then, the author follows through discussing the mythology and roots of yoga. Again, this provides important insight and an intellectual base into the spiritual aspects of yoga.