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 book where the impersonal teaching of the upaniṣads speaks directly of God as a Person for the first time: He possesses māyā, rules the world, and is known not by reasoning but by grace. Here too, for the first time in śruti, bhakti — devotion to God and teacher — is named.