Sonoma 14.4.1. System Settings > Internet Accounts > iCloud > Apps Using iCloud: Show More Apps. I have turned off iCloud sync for all the apps in the provided list (sreenshot #2), but for some reason I cannot turn it off for Books. Why is that? Is it a technical issue or a commercial (aka “by-design”) thing?
I just tried it. I can’t turn it off on any of the those apps in lower half of the Window (anything below Freeform). My guess is it is a bug in the Settings app, which I continue to despise. 
I understood that Books used CloudKit for its sync. But it may be able to use both CloudKit and iCloud Drive.