Ancient Sanskrit Yogic Texts
The yogis of yore had several texts or "reference material" to guide them in their practices. Most of these are hundreds if not thousands of years old and have been passed on from generation to generation by word of mouth.
Why are these texts relevant even today? Because they contain some universal truths. Because they throw light on the true aims and objectives of yog. And because they give a strong theoretical ground for the practical science of yog.
Unless you are an expert in Vedic Sanskrit, you will have to rely on a translation and commentary of the texts. You would do better to avoid translations by a modernist, especially Westerners who do translations after reading other translations. Some things about Indian culture have to be lived and imbibed and are very difficult to learn. These nuances are likely to be missed by non-Indians. Of course there are exceptions but generally you will do fine if you go to a translation done by a traditional Indian yog master who has been taught the Sanskrit version himself.
The Siva Samhita is the fundamental work on yog, said to originate from Lord Shiv, the founder of yog. In fact the first verse begins "Now the Lord Shiv, bestower of deliverance to all beings, expounds for the benefit of all his devotees, the Discipline of yog."
Chapter 1 is entitled, the Path of Harmony and gives an introduction to yog. Chapter goes into the fundamentals of Yog. Chapter 3 covers the system and its practices such as asan, nadi-shodhan, pranayam, while Chapter 4 goes into postures and preludes, especially Mudras and Bandhas. The last chapter is on Raja Yoga.
The Gherand Samhita is much more practical. It comprises of seven lessons the first introducing the six-fold system. The second lesson comprises of asans thirty two of them to be specific. Each asan is described how to perform it and its effects. Mudras or mystic exercise are taken up next twenty five of them. Lesson four talks about the control of senses, and lesson five about pranayam or breath control. Lesson six introduces you to meditation, while lesson seven talks about the aim of yog superconsciousness.
The Hatha Yoga Pradeepika by Swami Swatmarama is a much later text (16th Century) which is a practical manual for yog students. Chapter one deals with the pre-requisites of yog and the asans. Chapter two talks about Pranayam. Chapter three talks about mudras, bandhas and pranayam. Chapter four deals with Raj Yoga
The Patanjali Yog Sutra is a Raj Yoga text. It is at a very high level and difficult to understand for beginners. I for one have found it so. Though at least two thousand years old, it deals with the subject matter at a psychological, psychosomatic and metaphysical level. It comprises of 196 verses divided over four chapters covering the types of yog, practice of yog, powers of yog and samadhi or superconsciousness. To give you an idea of its terseness, the first verse reads "Atha Yog Anu-shasanam" Now (is explained) the discipline of Yog. Verse two defines yog "Yog Chitth Vritti Nirodhah" Yog is the restraint of the modifications of the mind. And so on and so forth.