Amazon may have pulled the plug on downloads to K4BNT. More details here:
https://www.mobileread.com/forums/sh...35#post4575235
ETA: I did more experimenting and discovered that I am still able to "deliver" books to my K4BNT by going to Amazon's website and going through Content Library > Books > Deliver or remove from device. Then when I go to my K4BNT and sync, the book will be delivered, though I had to restart it one more time.
However, if I navigate to the book on my K4BNT itself (Archived library) and click on it, it will not download.