For study of Ashtang Yoga (also called as Raj Yoga) we should study Patanjal Sutras. The books for study of Patanjal Sutras are
Patanjal Yogadarshan by K.K.Kolhatkar(Marathi)
Four chapters of freedom by Bihar school of Yoga
For study of HathYoga
Gherand Samhita & HathaYoga Pradipika are the text books. They are explained by Bihar School of Yoga, Kaivalyadhama – Lonavala.