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. You can tap on any ePubs you've saved here and import them.
3. Tap Locations to add more apps
If you tap Locations, in the top left, you can add Dropbox or Google Drive by tapping the More link. Any apps you have installed that Book Creator can access will be listed here. Toggle them on/off and then tap Done.
Now tap the Locations link again and you'll see these apps listed. You may need to sign in or use a passcode.
4. Find your book and import it
My book is in Google Drive, so I tap on Google Drive, find the folder I want, and tap on my file.
You'll then see your book loading in Book Creator as it imports.
From the blog: