Hey there Giuseppe,
It sounds like not all the covers of the books on your shelves are populating their covers. I would recommend starting by closing all the open and running apps on the phone:
iOS: Force an app to close
Double-click the Home button.
Swipe left or right until you have located the app you wish to close.
Swipe the app up to close it.
When you have done that, sign out of your iTunes account in Settings > iTunes and App stores, and restart the phone:
iOS: Turning off and on (restarting) and resetting
If the issue persists, I would next backup your device to iTunes, and restore it:
iOS: How to back up and restore your content
Thank you for using Apple Support Communities.