Carly Pugh and Shelby Day offer this very useful advice:
From Carly's post on the EDM310 Facebook Page
So for all of you designing an iBook for your final project, Shelby Day and I have found something to help! Since the My Sentence and other videos are required and iBooks Author "conveniently" doesn't have a function to install YouTube videos, there is a site to make your own widgets!! They have widgets for YouTube, Instagram, and many other options.
This is the site: Bookry
Sign up for a free account and you can make all the widgets you want! I highly recommend it. And also, for anyone who gets stuck, they have their own tutorials that tell you exactly how to make, install, and personalize it. *hint hint*