The Vedas and Upanishads give some of the earliest references to the paths of yoga. These scriptures form the basis of Indian religious practices but contain many varied references to yoga and other things.
There are the Puranas, also ancient, which deal with the nature of the universe. Famous epics such as the Ramayana and Mahabarata contain stories of the gods and lectures on moral and philosophical subjects with references to yogis and yogic practices.
The Bhaghavad Gita is a particularly famous part of the Mahabarata which contains a detailed discourse on yoga by Krisna to Arjuna.
Other texts such as the Hatha Yoga Pradipika are more “technical manuals” of yoga which go into detail on technique as opposed to just the theory.
In general all these texts discuss Yoga from the particular standpoint of the authors and the paths to Yoga they have followed. In many ways this subject can be confusing for lack of a clear overview. This need is answered in the Yoga Sutras of Patanjali.