If any chapter in the book module has an iframe that is pointing to a file index.html that is uploaded/linked in that chapter, then the iframe simply displays as blank and the user is forced to download an index.html file.
The index file downloaded is a HTML snippet of the book chapter displayed in Moodle and the iframe code looks like:
I noted this behavior when I upgraded a Moodle 2.7 site to v2.9 and some iframes in the book did not display. If you renamed the index.html to another name eg index2.html and update the src in the iframe, it displays without any issue. This only happens in the Book if iframe is pointing to a file named index.html. In all other places (page, assignment, forum) the iframes display regardless of the filename.
I can reproduce this on the Demo Moodle site as well. Screenshots are attached.