Books not syncing across iCloud

Yes, that fixed it. Mojave and iOS 14.4 will sync non-DRM non-Apple Store EPUB files even when the iCloud Apple ID is not the same as the Store Apple ID.

In my case the fix was to "buy" a free Apple Store Book, once that was done the "download new purchases automatically" setting in Mojave Book Preferences was no longer grayed out. I don't think I needed it on, but the purchase process fixed the disconnect that was disabling sync.

This article does claim non-store books will sync: https://discussions.apple.com/thread/251051568. But there's no iCloud sync setting for books in Mojave.

I also note that in preferences for my Mojave books "download new purchases automatically" is grayed out.

I just had a nasty thought on this. The settings only mentions synchronizing PDFs via iCloud. I wonder if it's possible that Apple does not support syncing ePUB files in Books.app! Perhaps ePUB will only sync if purchased from the Apple Store.

This particular ePUB is DRM free but not purchased from the Apple Store.

At first it seems bizarre that non-Apple Store ePUB would not sync, but that's how Apple Music works. Only tunes that are part of the Apple Store can sync (there is a mechanism for 'grandfathering' in old music however).

I have a bad feeling about this.

(I actually store all my library on a Google Drive and there's a way to manually move books to my iPad that way.)

I downloaded a free book from the Apple Store in Mojave and now "download new purchases automatically" is no longer grayed out. The free book did appear on the iPad. So that's promising. Now I need to find a book that's not in my Books library to test. Progress!