The Upaniṣads — the philosophical core of the Vedic tradition, revealing the nature of Brahman, the Ātman, knowledge, action and liberation. The ten principal Upaniṣads together with select minor Vaiṣṇava Upaniṣads.
On "what pertains to the self": who placed the soul in the body, what the false "I" is, and how to distinguish the witness from what he sees. One of the most tightly composed books on discernment.