1: In Settings App, go to iCloud
2: Press Storage
3: Press Manage Storage
4: In the Backups section - press your iPad or Device
5: Under Backup Options, make sure that unrealBook is set ON
In a few days, there will be a number below the unrealBook entry showing the size of the backup. It should not be zero. If it is zero, you are out of space in the iCloud backup storage. If you need to purchase more storage, choose Settings->iCloud->Change Storage Plan and make sure you choose a storage plan that has enough room to backup your unrealBook files.
Note that the only way to recover your files if you erase them with iCloud backup is to erase the entire device and restore from a backup. However, there is a product that claims to restore partial backups from iCloud. Worth a try.