If you are interested in a thorough, in-depth discussion of yoga philosophy, this is your book. The yoga sutras were written 1200 or more years ago and serve as the foundation of classic yoga. Iyengar interprets and expounds upon each of the 196 individual sutras. Each sutra is an aphorism or brief statement. The book is not for beginning yogis, but those interested in applying yoga to all aspects of their life will return to it again and again.
The Sanskrit glossary at the end of the book is a particularly useful resource.