ibooks
How do I move the content of iBooks on my old (OS9) iPad to iBooks on my new iPad (OS12)?
iPad, iOS 12
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Books (iBooks) content isn't backed up by iCloud - but you can download content again to your new device. On your new device, having ensured that you are logged-in to iCloud, simply access the Books App; all your content should be listed in the "purchased" section - and be available for download.
If you have restored a backup of your old device onto your new iPad, any content should also be visible from your Bookshelf.
Hope this helps.