This is a booktool plugin that allows the user to export their book as an EPUB v3.
The plugin code is available on GitHub from here: https://github.com/RichardPilbery/moodle-booktool_exportepub
It is based on the Generate IMS CP plugin and will appear as a link within the Adminsitration block once installed. All that is required is to click the link 'Create ePUB' and an epub version of the book will be downloaded.