Ashtanga Yoga is a practical everyday-use discipline based on Samkhya metaphysics/theory Worldview. Without both this background you will not understand the techniques; So i will start from there and give you the full picture;
0) For a general succinct overview of all the major schools of Indian Philosophy see An Introduction to Indian Philosophy by Satishchandra Chatterjee and Dhirendramohan Datta (read chapters on Samkhya and Yoga).
3) For a compendium of Techniques to use for Concentration (aka Centering) see Vijñāna Bhairava Tantra (https://en.wikipedia.org/wiki/Vij%C3%B1%C4%81na_Bhairava_Tan...). It briefly presents around 112 Tantric meditation methods (yuktis) or centering techniques (dhāraṇās) in very compressed form. The most accessible translation is the one by Ranjit Chaudhri while the most scholarly is the one by Jaideva Singh.
0) For a general succinct overview of all the major schools of Indian Philosophy see An Introduction to Indian Philosophy by Satishchandra Chatterjee and Dhirendramohan Datta (read chapters on Samkhya and Yoga).
1) For Samkhya (https://en.wikipedia.org/wiki/Samkhya) metaphysics see the slim Samkhya Darshan: Yogic Perspective on Theories of Realism by Swami Niranjanananda Saraswati. See also my previous comments on Samkhya starting here - https://news.ycombinator.com/item?id=41477432
2) For Patanjala Yoga Sutras (https://en.wikipedia.org/wiki/Yoga_Sutras_of_Patanjali) which lays out the framework of Ashtanga Yoga first see Bon Giovanni's succinct translation here (pdf) - https://www.starcenter.com/patanjali.pdf Then for a detailed study start with Edwin Bryant's translation The Yoga Sutras of Patanjali (with insights from the traditional commentators). To get an idea of practical application see Geraldine Coster's Yoga and Western Psychology: A Comparison - https://archive.org/details/in.ernet.dli.2015.189086
3) For a compendium of Techniques to use for Concentration (aka Centering) see Vijñāna Bhairava Tantra (https://en.wikipedia.org/wiki/Vij%C3%B1%C4%81na_Bhairava_Tan...). It briefly presents around 112 Tantric meditation methods (yuktis) or centering techniques (dhāraṇās) in very compressed form. The most accessible translation is the one by Ranjit Chaudhri while the most scholarly is the one by Jaideva Singh.