Desikachar, T.K.V. The Heart of Yoga: Developing a Personal Practice.
Desikachar is the son of Sri T Krshnamacharya, which is recommendation enough for many who are interested in yoga. Krshnmacharya, one of the most respected yoga teachers of his time, insisted that yoga practice must be tailored to the individual, not the other way around.
Beautifully written (and translated), The Heart of Yoga provides insight and understanding into the practice and purpose of yoga. Easy to read and easy to understand, Desikachar provides information about the history and philosophy of yoga, Patanjali's Yoga Sutras, the purpose of asanas, the importance of breath and movement, how to link asanas and why, and more.
It is not a book of poses and how to perform them, but a source of understanding more about the various poses, the difference between dynamic and static poses, the importance of taking into account the individual when designing a series of poses.
The individual takes precedence -- always. Perfect form in an asana does not mean that the pose is providing the service it should, and those who are less limber, less strong can modify poses to improve themselves physically and mentally. The yoga that Desikachar describes is not "fitness yoga" in the sense of intense exercise and beautiful bodies, but movements and philosophy that can benefit anyone and lead to improvements in health and well-being.
Nonfiction. Yoga. 1995. 242 pages.