Re: Unable to unsync Books to iCloud
Am no expert on iCloud / Books sync, yet your request surely deserves some reply.
You may need to follow the same procedure you used for turning on sync of Books,
to also turn off sync of Books:
Apple support's article: Set up iCloud for Apple Books on all your devices - Apple Support
Eg: says
how to turn on sync of Books
macOS 13.3 or later: Choose Apple menu > System Settings, click [your name]
at the top of the sidebar, click iCloud, click iCloud Drive, then turn
on Sync this Mac. Click Apps Syncing to iCloud Drive, then turn on
Books.
_____________________________________________________________
Another setting you may need to turn off in the same way it is turned on
See your books and audiobooks on all your devices with Books on Mac - Apple Support
says:
On your Mac: Choose Apple menu > System Settings, then click [your name]
at the top of the sidebar (if you don’t see your name, click “Sign in
with your Apple ID” to enter your Apple ID or to create one). On the
right, click iCloud, then click iCloud Drive (make sure it’s turned on).
Click “Apps syncing to iCloud Drive”, then make sure Books is turned
on.
Hope this helps :-)