The philosophy behind Yoga is deep and fascinating. It encompasses Epistemology, Metaphysics and Ethics.
The best known text on Yoga Philosophy is Patañjali’s Yoga Sutras, probably from the 4th century CE.
The Sutras are 176 short aphorisms. It is unusual in that there are no verbs. For example in Chapter 2,46 when talking about yoga poses, Patañjali simply says “steady comfortable posture” (स्थिरसुख॑म्आस॒नम्). It is left to us to understand that he probably means “Physical postures should be steady and comfortable”. Since the Yoga tradition has mostly been passed down orally with very little written, the fewer words the better.
There are many translations and interpretations of the Sutras. My favorite is Patanjali Yoga Sutras by Alistair Shearer. His translation is a delight to read.