This book includes Swami Vivekananda translation of Patanjali's Yoga Sutras.

This is a fine readable translation, but it's at a high-level.

If you are looking for the authentic and authoritative translation, I'd recommend:
~ Light on the Yoga Sutras of Patanjali by BKS Iyengar [faithful to the original text; practical/conclusive]

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This book provided the information I was looking for on Raja Yoga. Vivekananda's words are a pleasure to read. It has a nice commentary on Patanjali's yoga sutra. The book has a pronunciation guide page and the glossary was quite useful for me as well. As a result of reading this book I purchased Vivekananda's complete works.

Wow! What a treat! Swami Vivekananda has created a classic on Raja-Yoga that is head and shoulders above any that I have read in my 20 years of studying and practicing Advaita Vedanta. This is a great manual for those just beginning their investigation into the wisdom of Vedanta as well as very useful for those who have been practicing for years. Indeed, I would recommend this book as the starting place for any would-be practitioner. Swami speaks straight and clear and his advice is immediately understood and usable. This manual will be at my side for the rest of my life!

You can't go wrong with any book by Vivekananda. Not only was he a great scholar, he was a saint and mystic who had personally experienced the things he taught. This book gives an extremely clear description of Raja Yoga from both a practical and philosophical standpoint. The first half explains the philosophical concepts of Hinduism regarding the relationships between mind, matter, and spirit. The second half of the book walks you through the ancient Yoga Sutras.

Raja Yoga is the King of Yogas.
This book really enables you to understand how your mind processes information and how to conquer your own nature to exercise free will.
The author Swami Vivekananda was both an enlightened soul and an elequent speaker. His words are powerfull!