As I have mentioned on a few occasions, I am going to be teaching exegesis of the Gospel of John next summer and I am acquainting myself with Johannine scholarship (slowly!). One resource I have found immensely helpful is Jan van der Watt’s Introduction to the Johannine Gospel and Letters (T & T clark, 2008). At a concise 151 pages, it is remarkably comprehensive in its coverage of important topics, judicious in the treatment of controversial issues, and presents of all this in a way friendly to the non-Johannist (is that a term?).
This would make a great textbook for a John course – I am considering using it myself, though it will overlap significantly with any commentary. In any case, Jan models what it means to get to the heart of a text, trace its plot and development as a narrative, and search its theological depths. Do check it out if you want to learn more about John!