He talks about the logos and ancient concepts of harmony and symmetry, Greek numerology and many Gnostic concepts become very clear without the need to even refer specifically to the Gnostics.

Pagels ultimately fails to be the most compelling writer because she uses Gnosticism to talk about other points of general theology that interest her, rather than discuss Gnosticism on its own terms.

Of course, ultimately the best thing to read is the Nag Hammadi codices. Bentley Layton from Yale has a good annotated version, and if you want to delve deeper, there's always the Brill academic version.