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.
The "unsupported" book: a series of questions and short answers on who Brahman is, what caste is, what action is. It ends with all this being known not by one's own mind, but by the grace of a teacher.