This plugins requires gitbook >=2.0.0.
Add it to your book.json, then run gitbook install:
{
plugins: ["youtube"]
}
You can now add youtube vieo to your book:
take a look at this video:
{% youtube %}https://www.youtube.com/watch?v=9bZkp7q19f0{% endyoutube %}
The video will be replace by the embed iframe in the website and by a link in the ebook versions.