Lost my books on iPad

Hello, I lost all my saved books. happened after disabling and enabling synchronization with icloud, but when restoring the application volume is more than 9 gigabytes, but the books are still not visible, how can they be restored?

Ps

Do you have another iOS device like iPhone or even a Mac? You can install Penbook on that device and see if the sync works there, then we can narrow down the problem.