iBooks are not synchronized via iCloud between my Apple devices

Solution 1:

• Open Settings app on both devices and verify both have an iCloud backup (or connect to iTunes and back up both)
• Sign out of iTunes & App Store and then reboot both devices
• Sign back in to the iTunes & App store on one device
• Open iBooks and see what it shows for your purchases / books.
• Tap to download one of each (a purchased book and a synced book)
• Then review the iBooks settings in the settings app for this device

Repeat the sign in steps for the second iOS device. Pay attention to the "Sync Collections" setting for iBooks.

Solution 2:

I've encountered this a couple of times and the following always gets things working again for me.

1. Open System Preferences, click Apple ID
2. From the list of "Apps on this Mac using iCloud", select "Options" for iCloud Drive
3. Uncheck Books, then click Done
4. Repeat steps 2 and 3, but this time check Books again

My iBooks library synced immediately after this. I'm running MacOS Catalina.