If you have upgraded your iPad to IOS 12, and now your iBooks are missing, try these fixes [taken from a variety of locations across the web}:

1. Check for another system update in Settings –> General –> Software Update. [Student iPads –> This has to be done at home.]
2. Do the iPad Reboot Dance.
3. Redownload the books. In iBooks, click the link “Purchased”. Choose the ebooks you need to recover.
4. Redownload from iCloud. assuming iCloud is set to backup your iBooks, then this might work.
   Settings –> AppleID (at top left) –> iCloud.
   Swipe down and turn the iBooks switch OFF.
   Wait 30 seconds.
   Do the  iPad Reboot Dance.
   Back to Settings –> AppleID (at top left) –> iCloud –> iBooks.
   Turn the iBooksswitch ON.
   Check iBooks.
5. Offload the iBooks app.
   Go to Settings –> General –> iPad Storage –> scroll down to iBooks.
   Tap OFFLOAD APP.
   Shutdown, then restart the device.
   Go back intoSettings –> General –> iPad Storage –> and press REINSTALL APP.
   Check iBooks.
   If the books are still not there [or aren’t loading], then reboot your device again.
6. Toggle iBooks Sync Off, then On.
   Settings –> iBooks –> Sync Bookmarks and Notes.
   Open iBooks.
   If they haven’t shown up, then close iBooks, change it back to ON, and check again