If you want something that is a scholarly introduction, I can recommend "Introduction to Gnosticism: Ancient Voices, Christian Worlds" by Nicola Denzey Lewis. It also has some material on pagan and polytheistic Gnosticism, since some of those texts were recovered at Nag Hammadi, too. I found it to be a very good scholarly introduction to Gnosticism. Not as good, but still recommendable, is "The Gnostic Bible: Gnostic Texts of Mystical Wisdom from the Ancient and Medieval Worlds," Willis Barnstone and Marvin Meyer as editors. A nice collection of Gnostic works, with good introductions for each text and school of Gnostic thought.