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.
It divides "the person" into three: the body, the inner agent, and the supreme Ātman — and shows what remains of the three. A short book, from which one sees how the whole dispute over "I" is built.