If you're using Book Creator online, you can save the ePub file directly to your computer. Then it's just a matter of uploading that file or syncing with a cloud service to back up the book remotely.

In Book Creator for iPad, let's look at how you use the Files app to save books either locally on your iPad, or (even better) backing them up to a cloud drive such as iCloud Drive, Google Drive or Dropbox.

WARNING: We cannot stress enough how important it is for you to backup your books regularly. Books made in Book Creator for iPad are stored locally to the app on that device, so if you delete the app, or the iPad is damaged or lost, there is no way to recover your books unless they have been backed up to a cloud service.

See also:

Make sure you have installed the cloud app you want to used, signed into your account and then added access to the Files app on your iPad.

Here's how you do that

On the bookshelf, tap the Share icon beneath your book (on the right) and choose Exporta as ePub.

Choose the Save to Files option and then navigate to the folder where you want to save your book. Note - you could also directly tap on the icon for the app you want to share to. You could also choose 'Open in...' or 'Send to...' and it would do the same thing.