Traces the use of powerful gnostic visionary techniques from Hellenistic Gnosticism and Jewish merkabah mysticism, through Muhammad, the Ismaeilis, and theosophical Sufism to medieval neoplatonism, and renaissance alchemy.
If you're interested in the gnosticism of early Christianity, you'd probably be better served with Elaine Pagels' more accessible work. Merkur tends to be tangential and probably a little more intensively detail-driven for the lay reader. I say tangential because Merkur strays unnecessarily into weird cul-de-sacs, like alchemy and gold magic, as well as odd detours into discussions of heaven numerology and the Ogdoad. If he'd stuck to his guns and fleshed out the historical bits a bit more than the strictly "philosophical" bits, it would've been a stronger work. He does make excellent connections with Islam and especially Sufism while tending to be more cautious in linking these traditions with their Christian counterparts. He is somewhat dismissive of scholars like Corbin and Eliade, while also neglecting recent neuropsychological work that shows connections between feelings of all-is-one universalism and things like epilepsy and funniness in the brain's limbic system.
As far as i can see, Merkur follows a reductionist storyline. Mystical experiences are intrapsychic events without correspondent outer realities. However, his systematization of visionary techniques is worth reading.