Using a cloud service like Google Drive, Dropbox, or iCloud Drive is a great way to back up your books, or collaborate across iPads and bring them together into one book.
You can always open a native app like Dropbox, choose the ePub file and tap the export icon. Choose 'Open in...' and you should see Book Creator as an option.
But here's a way to do it all from within Book Creator.
1. Tap the + symbol on the My Books screen
2. Now choose Import Book
You'll be presented with two options.
Copy from iTunes will show you a list of any books you have exported to iTunes, or added via iTunes on your Mac/PC.
Tap iCloud Drive. You'll see the iCloud Drive folder system that was introduced with iOS 8 (or you'll see the Files system if you're using iOS 11 or higher). You can tap on any ePubs you've saved here and import them.
If you don't see the app you want listed - you may need to open the Files app on your iPad and toggle it on in Locations first (see below).