The book Astanga Yoga is not an independent work of Swami Charandas: it is a part of the book Bhakti Sagar. As said above, Astanga is one of the chapter of the same book. It covers all 8 steps of Patanjali Yoga: Yama, Niyama, Asana, Pranayama, Pratyahar, Dharana, Dhayana and Samadhi.