The Yoga Sutras of Patanjali is the original yogic manuscript. All forms of yoga that we know today stem from this root. It is a deeply philosophical piece that takes time to digest.
By reading the origin text of yoga, you can gain a very deep appreciation for the depth of yoga. This is the Bible, the Torah, the Quran of yoga. It contains a message with the deepest truths and goals of the practice.
Paramahansa Yogananda is the first major teacher of Indian Yoga in America. He introduced millions to yoga and meditation, was hosted in the white house, and had 100's of Kriya yoga centers established around the world. He is a true pioneer of bringing yoga to the west
In this book, His Holiness the Dalai Lama describes fundamental principles of Buddhism. Some of these teachings include: how individuals can transcend suffering via meditation and how karma works. This is wisdom on the fundamentals of the meditative path from one of the most dedicated meditators in the world. The knowledge in this book is surely worth more than the $10 it costs.
B.K.S. Iyengar is one of the most prominant yoga teachers the world has ever seen. He was listed in Times Top 100 most influential people, and won prestigious awards in India. He practiced yoga asana's for hours a day, even into his 90's. His level of awareness of the body and mind is seemingly impossible to me, growing up the United States. He understands yoga to a depth that is unknown to many in the west. So by reading this book, you can unlock the depth of yoga.