The principal Upaniṣads

The end-portion of the Vedas, and a change of subject: away from ritual and toward the question of what a person fundamentally is. Dialogues in forests and courts and kitchens — a father and son with a cup of salt water, a king and a sage, a boy who waits three days at Death's door and gets three questions for it.

The source everything in this pack argues about. Śaṅkara, Rāmānuja and Madhva all claim these texts and reach three incompatible conclusions from them — which is the best possible evidence that they are richer than any of the three.