Texts

There are many great texts on the subject of Yoga. Below is the list of my most cherished and influential Yoga texts. Of them all, The Yoga Sutras of Patanjali is perhaps the most important and applicable known text on the instruction of Yoga. This text is much more advanced than the physical practice. This falls under 'Raja Yoga' (Yoga of the mind). Many different translations exist. Four Chapters on Freedom is the best version that I have found on the Yoga Sutras. Further information regarding Bihar Yoga and their other texts can be found here. Additionally, here is a free version of the Yoga Sutras.

Yoga of the Mind
Four Chapters on Freedom by Swami Satyananda Saraswati

Bhagavad Gita
Upanishads
Crest Jewel of Discrimination
The Divine Life Society

Free Online Books by Swami Krishnananda

Physical Yoga

The Key Muscles of Yoga: Scientific Keys, Volume I
The Key Poses of Yoga: Scientific Keys, Volume II
The Science of Yoga: The Risks and the Rewards by William J. Broad

Master Chart of 908 Postures: Of and By Sri Yoga Dharma Mittra

Assorted Yoga and Vedic Texts

Hatha Yoga Pradipika by Swami Muktibodhananda

Moola Bandha: The Master Key by Swami Buddhananda

Yoga Nidra by Swami Satyananda Saraswati

Vedanta Treatise: The Eternities by A. Parthasarathy