Books Won't Sync or Import M4B Files

Recently Upgraded to Sonoma. Prior to upgrading, I had a very large (several GB) collection of personally recorded audiobook (M4B) files in Books, synced via iCloud across an iMac, MacBook Pro, iPhone 15 w/iOS 17, and iPad Mini w/iOS 17, as well. All sync settings seem to be set the same as before I upgraded and I have been using this configuration for several years.

When I opened Books recently, for the first time after upgrading, all of my M4B files are gone on my iMac. When I checked my MBP, which is still running Ventura, as it is too old to upgrade, the same thing - all M4B files are gone. On both the iMac and and the MBP, a couple of dozen of commercially purchased books and audiobooks are still there. All that I had there, I think.

Both my iPhone and my iPad still appear to have my full library with no issues.

The other wrinkle to this as I have tried to import a new self-recorded audiobook in M4B format into Books on my iMac, but it does not import and shows "Failed" in the import status dialogue.

I tried once to uncheck the iCloud sync setting for Books on my iMac and redo it to start a new sync, but it did not appear to have any effect and I'm a bit hesitant to try further as I am a bit worried that the lack of these files on my iMac will somehow delete them in iCloud (where I hope they are still located) or on my mobile devices.

All insights appreciated, starting with how do I confirm that these still exist in iCloud, in addition to what I appear to still have on my mobile devices?