Pressbooks supports adding one sub-level to your Table of Contents. Below are instructions on how you can use Heading 1 to enable a second level on your TOC.
- Go to Appearance > Theme Options
- Select “Enable a two-level TOC”
- Now, in your content, headings tagged with <h1> tags should be parsed into a (linked) subsection in your TOC.
Please note that these second levels (with H1 headings) appear collapsed by default. This means that while they are in the TOC, you will have to click on the + button next to the chapter so that this second level shows.
Finally, not all Themes have been tested with this feature — if things look funny, please test another theme.