Often the first book read by the aspiring student of gnosticism, Pagel's "The Gnostic Gospels" is a classic for a reason. This study carefully teases out the various threads of gnostic heresy, and explains precisely why gnosticism was heresy through the writings of the early theologians (Tertullian, Irenaeus, and others). Gnosticism presented a form of Christianity that had great intellectual appeal and threatened the institutional form of the religion already taking root in the early church through it's focus on self-realization of divinity. To this day, mystics and theologians walk a tenuous line between orthodoxy and gnosticism, especially when questioning where faith ends and experience begins. Pagels clearly deliniates the various theological struggles between the gnostic sects (many of whom were very different from each other) and the universal church, and how theology and power collide- and how they do not.
Pagel's study is not without it's flaws. It is quite brief, and does not cover gnostic theology or studies with real depth beyond the conflicts of doctrine. It's also expected that the reader is already familiar with the gnostic texts- this is not an overview of gnosticism. But other than that, I highly recommend this book to anyone who wants to study gnosticism and it's discontents.