Nine Principal Upanishads
Swami Satyanananda Saraswati,a disciple of Swami Sivananda founded the famous Bihar School of Yoga in 1964.A prolific writer he is the author of some of the outstanding Books on Yoga like Asana Pranayama Mudra Bandha,A Systematic course in the Ancient Techniques of Yoga & Kriya and many other Books.  Most of his Books are best sellers and are listed on amazon.com.