Click the Add child link on any existing Book page that is contained within a book structure.

Title field = enter a title

Body field = enter body text

Click the Save button.

After the node is saved, you'll see that the Book navigation menu automatically updates.

You may also see additional navigation links at the bottom of your page - these can be turned on and off in the theme settings.

Returning back up to the parent page, you can see that this page has been affected as well. A table of contents will start displaying when a child page is create beneath its parent.

Clicking on the Add child page for subsequent children will add additional children to a structure. Below we have an example of a book structure that has two children and one grandchild.