If you embed the book under a https website, you need to use the book https url
You just need to modify the book url to https format:
For example,
1. Your flipbook’s http URL : http://online.pubhtml5.com/ylxu/fbsb/
You just need to change it to this format:
2. Your flipbook‘s https URL : https://s3.amazonaws.com/ online.pubhtml5.com/ijua/bmqh/ index.html