I loaded up a bunch of travel books on a 32GB iPad for an overseas trip this summer only to have them all evaporate shortly thereafter. Every time I got on decent WiFi I would redownload and reimport them from iCloud Drive. Sometimes they would stick around for a few days and sometimes they would be gone in literally hours (gone from the device, not from iCloud Drive).
Incredibly frustrating if you have big reading plans for your offline time. My understanding is that there's no way to get the books to stay put without disabling iBooks iCloud syncing and then loading or importing them the old fashion way (from Dropbox, or via iTunes etc).
I think it was ".nosync" and ".tmp"
https://forum.keyboardmaestro.com/t/exclude-files-folders-fr...
Also "Bailiff":
"Bailiff is a simple menubar app which lets you evict iCloud files and folders from local storage, or download them when you want. Saves your Mac’s startup disk from getting cluttered with files you don’t want or use."
At least if it was by creating an empty file with that name.
Also iCloud Drive's synching has been an utter mess and I gave up after nothing syncs at random times.
That's not very nice. I'm just trying to help and that seems to be the only thing that seems to be working.
I don't work at Apple, so...