Showbie is unique among classroom apps in that they specifically developed the app to include the ability to preview Book Creator ePub files.
So here's how you export your book from Book Creator for iPad and read it in the Showbie app.
Tap the Share icon beneath your book on the My Books screen
Choose Export as ePub
Tap on the Showbie icon
Choose the Class to post to, then the Collection, and choose which students to share with
Tap Add File
Wait for the file to upload to Showbie, then navigate to it
Tap on the title and it will download to Showbie, allowing you to read it - turn the pages and play multimedia!
Learn more from Showbie
This video from 2017 shows some more ways in which Book Creator and Showbie can be used together.