seconded on Patanjali This is what any practitioner needs to read or you'll have no clue what yoga is even about unless you read some even more ancient vedic texts or something. But Patanjali incorporates the important stuff from all yogic traditions.

Depending on how deep you want to go, you can read several translations. Every sanskrit word of these sutras is laced with deep meaning. But it is very manageable if you start with a nice, accessible, middle-of-the-road translation. I like this one: https://www.amazon.com/Yoga-Sutras-Patanjali-Weiser-Classics-dp-1578637309/dp/1578637309/ref Mukunda Stiles made a great, poetic translation, conveying the basic concepts well to the layman.

It may not be as readily accessible to the western mind as a translation like this: But at the same time, it doesn't try to hammer square pegs into the round holes by constant allusion to christian theology, and doesn't block the flow of the sutras with the author's interpretation.

It's not a super-advanced translation where every term is expounded upon like some more academic-focused authors, turning what should be simple aphorisms turn into paragraphs of explanation. For that same reason, it doesn't get in its own way. Highly recommended.