Via Antonio Lombatti and Jim West, I became aware of an article on the background to the Gospel of John. It is by Vhumani Magezi and is in the open access journal based in South Africa HTS Teologiese Studies / Theological Studies, 66:1 (2010).
I’ve only read those sections that discuss the Mandaeans, but was disappointed to find the author assuming that it is an adequate solution to the issue of the relationship between Mandaean sources and the Gospel of John to assert that the former borrowed from the latter. Magezi draws on C. H. Dodd’s outdated treatment of the Mandaeans, and an insulting description of the Mandaean religion offered in some sort of pamphlet produced by the Bible Institute of South Africa.
I’ll have to try to make a point of proposing a paper for SBL revisiting the question of Mandaeism and the Gospel of John at some point in the near future.