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 most orderly exposition of Advaita in the whole collection: four chapters on how the world unfolds from the unmanifest, and how a person turns back. We read it closely and dispute it on the merits, not over trifles.