A student of mine has just returned Alastair Logan's book, The Gnostics, to my library. She had borrowed it from me a while ago when I had first received it in the mail, and before I had a chance to read it. So now I am doing so with great pleasure.
I highly recommend Logan's book, which in part is a response to many North American scholars who have been attempting to purge the Academy of the Gnostics, at least as a category.
Although I think that the North Americans have made a good point - that Gnosticism was not a religion, but is a modern category - I have chosen myself to continue to talk about Gnostics and their diversity. As I explain in several of my articles, and again in my book The Thirteenth Apostle: What the Gospel of Judas Really Says, the Valentinian Gnostics were a group that was within the apostolic church system. Although they met privately in some type of lodge or school setting, they also attended the apostolic churches, joined in their rituals, and interpreted scriptures in similar fashion. The Sethian Gnostics (who wrote the Gospel of Judas among many other books that we have preserved at Nag Hammadi), however, defined themselves outside and even against the apostolic churches. They met privately, had their own rituals, and engaged in reverse exegesis, reading scripture in opposite ways of the apostolic Christians.
So for me, Gnostic is a very useful term, as long as we don't lump everyone in the same pot. This means that I am really glad to now be reading Logan's take, which appears to have many overlaps with my own.
Logan uses sociological theories to distinguish between cults and sects, and argues that the Valentinians were a schismatic movement or a sectarian movement within Christianity, while the Gnostics (=Sethians?) were a cult functioning outside the Christian Church. I am not convinced that the origins of the Gnostics (=Sethians?) was in Antioch at the end of the first century. Against this proposal of Logan, I would trace their origins even earlier and to Alexandria. The last chapter is very intriguing, since Logan wonders aloud whether or not the Hypogeum of the Aurelii in Rome is a Gnostic burial site.
If you haven't seen this book, but are interested in all things Gnostic (as I am), this is a must-read summer book for your list. It is short - only 150 pages - but engaging in so many of the right ways.
Update 6-17-07: Judy Redman posts on Thomas and Gnosticism in relation to Logan