You may know this already, but Duane is, supposedly, working on that latter, and has put out a few additional works in service of writing it, but I don't know how progress is on the actual novel. More info here.

The additional works:

https://www.dianeduane.com/portfolio/the-levin-gad-tales-of-the-five-1/

https://www.dianeduane.com/portfolio/the-landlady-tales-of-the-five-2/