Book Creator books are automatically saved so you can access them every time you log in. But you may want to save the ePub file to back it up in a cloud app, or to send it to someone else.
This method is particularly useful if you want to transfer your book from Book Creator for Chrome to Book Creator for iPad.
Find the book you want to share.
Click on the Share icon beneath your book, and choose Download as ebook.
The book will automatically download to your computer (wherever the default location for downloads is). It will save as a .epub file.
If you want to save it to a cloud app like Google Drive, Dropbox or OneDrive, you'll have to manually move it from your computer.
To share the book, you could email it, or send a link to the ePub file in your cloud app.
Please note, ePub files don't support transparent images and page elements and so they will all be downloaded at full opacity.