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.
Twenty-five verses on contemplating Śiva in the heart: solitude, even breathing, renouncing possessions — and the vision of the One called by many names. One of the clearest books on this state being accessible to the householder too.