Best Practices for Creating Interactive Textbooks

Hi,

I'm new here. I work for a college that produces its own textbooks, and I'm looking into creating more interactive ones. We use reflowable epubs exported from InDesign and currently our interactivity is limited to different hyperlinks taking you to key words, learning objectives, etc. in the text.

I'm just wondering if there are any best practices or common interactive features we're missing. I know this is a bit vague but that's kind of because I think there has to be more we can do so any tips are welcome.