by Gurudev Sri Sri Ravi Shankarji
Recommended
This is a must have book for those seeking to understand Patanjali's Yoga Sutras. Gurudev's explanations are lucid and accurate without sacrificing any details. The numerous anecdotes included make it engaging and enjoyable to read. It also features Q&A sections addressing frequently asked questions. The sutra translations on the site are largely from this book. However, the literal sutra translations are simply not enough to grasp the depth and nuances of the sutras. So I would strongly recommend purchasing this book. Available on Amazon, Art of Living Store and other online bookstores.
by Sri Swami Satchidananda
This is another well respected and popular book on Yoga Sutras, especially in the west. Available on Amazon
This site has a unique way in which the sutras are organized visually so you can get to any sutra quickly while also knowing what the sutra is about.
Another good site, has break up of individual sanskrit words and translations in multiple languages. Also has audio for each sutra.
Full text is available on this site.