First of all, a big thank you to @audiamus for a truly great program!

In the last few days I found that I could no longer download multipart books. I set logging to the maximum level (4), and read the log. I am in the process of anonymising the relevant part of the log, but I think that the problem is already clear from the following line:

10:11:12.628 0001 [AudibleApi.sendForStringAsync] HttpRequestException: "Response status code does not indicate success: 404 (Not Found)."
{"message":"Audio Part asins are no longer supported."}

I wonder if this explains some other users' problems as well?