Tripura Upanishad is one of the Upanishads in the Rigveda. Here you can download Tripura Upanishad PDF document for free. This Upanishath explains about the ‘Tripura’ (three cities) in human body and human energy. Body, Conscious Mind, and the unconscious mind are the three cities associated with senses or actions, and breath, and the spiritual energy.
Tripura Upanishad also details about the cosmic energies of the Mother Goddess.
Here is the meaning for a verse from Tripura Upanishad:
Three cities (Tripura) are there, and pathways three for all and the letters are a, ka, tha and others. In them there dwells, never-aging, ancient, the exceeding grandeur of the gods.