Three Picks:

Pagels, Elaine The Gnostic Gospels (New York: Vintage Books, 1989; originally published 1979)

Hultgren, Arland and Steven Haggmark (eds.) The Earliest Christian Heretics: Readings from their opponents (Minneapolis: Fortress Press, 1996)

Holroyd, Stuart The Elements of Gnosticism (Rockport: Element, 1994)