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 the "swan" — the soul, which flies between breath and thought — and on the inner sound heard in silence. The book is short and almost entirely about hearing: ten sounds, from a buzzing to a thunder, and what to do with the last of them.