Yoga Sutras of Patanjali is an essential sacred text of understanding Yoga. Without proper understanding and scriptural knowledge, Yoga can not be comprehended. Yoga is not simply Asana and it never will be. Yoga is the path of Self – Realization and it will stay that way for the true seeker.
There are various translations of Yoga Sutras, my Guru Baba Hari Dass translated them, but there are also other good versions, so feel free to find the one which suits you. This text is of most importance to a true Yogi.