First be re-assured your books are no permanently lost, Apple remembers what you purchased and they can be downloaded again without charge.
Second: assuming you backup your iPad either to iCloud or to a computer, as all wise users should, you can restore them from there.
iOS and iPadOS files are stored in one of two possible locations either Encapsulated within the application itself or on iCloud in a folder with the same name as the Application. So just to be sure, in Books there are three places books can be found, Books your are reading now, your Library, and the Book Store. Within the Library you find Collections
and the number of books in each collection. I know this is elementary, but be sure you have selected a collection that has some books in it. It is an easy mistake to make.
If everything still indicates no Books then go to Settings > General > iPad Storage > Books touch the > symbol and you will find two listingsâ€¦
- The App Size (36.8 MB on my iPhone)
- Documents & Data (Books stored on your iPad)
The space is occupied by Documents & Data is the best indicator of books physically stored on your iPad.
- If the Documents & Data size is very small or zero that would indicate that by some mischance they have been deleted or lost. In that case your best and easiest recovery method is to restore your iPad from the last backup following these Apple instructions.
- If it appears your documents have been lost and you do not have a backup then your only option is to be sure you are logged on with the same Apple ID and password you used to obtain the books in the first place and download them from the Apple Book Store once again.
- If the size for Documents & Data indicates the books are still present on your iPad and you are running iPad OS 14.x or 15 then in Settings > General > iPad Storage > Books choose the option to Offload App and when that completes choose the Restore App option that appears. DO NOT DELETE THE APP as that will also delete any data. (This worked for me on more than one occasion.)
Let us know what happens.