According to Lewis Spence in An Encyclopaedia of Occultism, the Oupnekhat or Oupnekhata (Book of the Secret) is a work written in Persian providing the following instructions for the production of visions:
"To produce the wise Maschqgui (vision), we must sit on a four-cornered base, namely the heels, and then close the gates of the body. The ears by the thumbs; the eyes by the forefingers; the nose by the middle; the lips by the four other fingers. The lamp within the body will then be preserved from wind and movement, and the whole body will be full of light. Like the tortoise, man must withdraw every sense within himself; the heart must be guarded, and then Brahma will enter into him, like fire and lightning. In the great fire in the cavity of the heart a small flame will be lit up, and in its center is Atma (the soul); and he who destroys all worldly desires and wisdom will be like a hawk which has broken through the meshes of the net, and will have become one with the great being." Thus will he become Brahma-Atma (divine spirit), and will perceive by a light that far exceeds that of the sun. "Who, therefore, enters this path by Brahma must deny the world and its pleasures; must only cover his nakedness, and staff in hand collect enough, but no more, alms to maintain life. The lesser ones only do this; the greater throw aside pitcher and staff, and do not even read the Oupnekhata. "
This book is possibly a revision of one of the Hindu Upanishads. Oupnekhata is probably from a nineteenth-century German translation titled Das Oupnekhat; die aus den Veden zusammengefasste Lebre von dem Brahm (Dresden, 1882), derived from an earlier Latin edition of 1801.
There is no single Upanishad "Book of Secrets." All the Upanishads contain the esoteric wisdom of Hindu metaphysics (derived from the Vedas. ) Comparable forms of meditation are also found in various Hindu yoga treatises and in the Bhagavad-Gita, a Hindu scripture derived from the Mahabharata, a religious epic.