Swami Vivekananda 
Râja Yoga [PDF ebook] 
Being Lectures by the Swami Vivekananda, with Patanjali’s Aphorisms, Commentaries and a Glossary of Terms

This book, first published in 1922, examines the science of Râja Yoga. All the orthodox systems of Indian philosophy point to one goal, the liberation of the soul through perfection – and the method to attain this is through Yoga. This book presents lectures on Yoga, delivered to a western audience view to explaining Indian philosophy; the lectures are accompanied by the Sutras (aphorisms) of Patanjali, along with an explanatory commentary.