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 prayer beads: what they are made of, how many beads, how to hold them, and what each letter on them means. One of the few places in śruti where this practice is described in such detail.