Hatha Yoga Pradipika
Famous yoga text/book. It translates from Sanskrit as Light on Hatha Yoga (i think..need to check this). In this text, Hatha yoga is discussed deeply. Explanations as to what are Asanas, Mudras, Bandhas, Mantra invocations and more. Also emphasis on purificatory practices. The Shat Karmas which are 8 purification techinques to keep the organism healthy.
Popular reproduction of this text is published by the Bihar School in Mysore, India. (link will be injected later)