iOS - Archive/Restore Library
Archiving and restoring iOS's Apple Books library is fairly straightforward. All the relevant data exists in a single folder. A simple drag/drop, copy/paste or zip/unzip should suffice.
When restoring a library make sure that the current version of iOS and the version the archive was created from are identical. Not doing so could lead to unexpected results.
See iOS - Library Location and iOS - Access Library for more information.