It is an academic work, not for the usual person just interested in "gnosticism".
The book demands you already knows the majority of themes involved in the "christian gnostics" of century I to III... the Nag Hammadi codices, for sure.
The author is completely right about calling "gnosticism" a "religion", "sect" or "movement"... All that exists is a collection of texts, some with certain affinities (like the Demiurge or the "anti-cosmos" attitude), but nothing organized.
I , personally, think the "gnostic" seed of knowledge comes from another pratices, including shamistics ones, of self-knowledge, applied to the "hype religion" of their particular moment (The Christianism).
As what happened with budhism in Tibet, with the old Bon religion mixing with the Sidartha teachings... gnostical pratices mixed with christianism, producing what the nowadays scholars call "Gnosticism", as a category to facilitate the learning and study of the phenomena.
Probably, the real origin of "Gnosis" will never be found, maybe because it was oral traditions, not writing ones.
Anyway, I recommend this book to academics and to people really into "Gnosticism".