Books are missing from mobile device (iPad) after syncing; only placeholders are shown.

I have all my E-books stored on my Mac. In the past, I haven't had a problem with them syncing with my mobile devices (which I do over USB). I add all of these local files to Books on my Mac.

On a recent trip I found all of my books missing from my iPad. On attempting to access any of them, it said "this book can't be downloaded." But none should have been needed to be downloaded, since I've synced all of them from my computer. This has worked in the past. I don't know why they're all missing now, with useless placeholders displayed in the Books app on iOS.

Update: In the past I had to activate iCloud syncing for Books simply to get collections synced across devices; this must not have worked when syncing over USB. That activation appears to have disrupted the syncing functionality. Compounding the bug is the apparent lack of any "download all" function in Books.

Deactivating Books's use of iCloud, deleting all Books data from iCloud, then deleting and re-adding all my books to Books on the computer (from local E-book files) appears to have "fixed" the problem for now.

Putting this here for anyone else who encounters this bug.