Books Download Failing on Big Sur

For their past several weeks books purchased from within the Books app have downloaded to my iPad but fail to download to my Mini running Big Sur. They are visible on the Mini, complete with the iCloud icon, but when clicked the download fails. That is the sum total of the message I get.

Going in the opposite direction works fine. I used the Mini to get an Apple manual, and that also downloaded onto the iPad as usual. I’ve restarted, signed in and out of iCloud, Books Store, and the Music Store in various combinations. I’ve trashed everything I could find that seemed related to caches for Books. I’ve confirmed that the epub files themselves are not actually on the hard drive but hiding. I can download the volumes in question to my phone. Music that I’ve bought over this same period does not have this issue.

Does anyone have any thoughts about what I could poke at next?

Dave W.