This text translated by Sri Swami Satchidananda is essential to any serious yoga practitioner.  It maps out the practice in four books of aphorisms moving the student from the gross to the refined.  The Sutras explain the science of yoga and its eight-limbed system.  What I love about the book is that you can read a bit and study and apply it and then continue onto another concept.  It is a helpful guidebook to explain this rich spiritual practice and translate this ancient tradition of yoga into practical terms for us today.  You can buy it online or in many yoga studios that sell literature or check a good book store.