"The Upanishads (/uːˈpænɪˌʃædz, uːˈpɑːnɪˌʃɑːdz/; Sanskrit: उपनिषद् Upaniṣad [ˈʊpɐnɪʂɐd]) are late Vedic Sanskrit texts of religious teaching and ideas still revered in Hinduism. They are the most recent part of the oldest scriptures of Hinduism, the Vedas, that deal with meditation, philosophy, and ontological knowledge; other parts of the Vedas deal with mantras, benedictions, rituals, ceremonies, and sacrifices. Among the most important literature in the history of Indian religions and culture, the Upanishads played an important role in the development of spiritual ideas in ancient India, marking a transition from Vedic ritualism to new ideas and institutions. Of all Vedic literature, the Upanishads alone are widely known, and their central ideas are at the spiritual core of Hinduism."

          (from Wikipedia)

     English translation of the Upanishads by F. Max Muller