Charles Johnston 
The Yoga Sutras [EPUB ebook] 

According to Wikipedia: ‘The Yoga Sūtras of Patañjali are 196 Indian sūtras (aphorisms) that constitute the foundational text of Rāja Yoga. Yoga is one of the six orthodox āstika schools of Hindu philosophy, which, according to the Yogatattva Upanishad, is divided into four forms – Mantrayoga, Layayoga, Hathayoga and Rājayoga – the last of which is the highest (or royal) practice. Although the Yoga Sutras have become the most important text of Yoga, the opinion of most scholars is that Patañjali was not the creator of Yoga, which existed well before him, but merely a great expounder.’