You basically make a manual backup of your unrealBook files and save them to your computer. Then you copy the backed up unrealBook files to the other iPad. There are two programs that work – iFunBox and iMazing. You do not need to purchase unrealBook again if you use the same AppleID for the new iPad and App Store.
Here is how to copy all unrealBook files to another iPad using iFunBox – which is a free program.
Here is how to copy all unrealBook files to another iPad using iMazing, which is a paid program.