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.
A book on renunciation: where to seek the holy place (within, not on a map), when a person has the right to leave home, and what one who cannot perform the rites should do. Hence the rule: one leaves not by age, but "on the day dispassion has come."