The fastest and most reliable way is to use iTunes (prior to Catalina).
This method will backup only the unrealBook music files and works well.
The following methods require you to restore the entire iPad if you need to recover the backup:
Make a complete backup of your iPad via iTunes.
However, if you are running Catalina, this is no longer easy or possible to do.
Therefore, use iCloud backup or iFunbox (mentioned above):