To add footnotes that will appear at the end of the current page you need to insert a shortcode into the book text like this:
The Footnote shortcode displays a supertext number next to the word immediately to the left of the footnote shortcode and then adds a note at the bottom of the current page.
Edit the text within the speech marks to be whatever you want to appear in the note. An example of three shortnotes added to text would look like this:
This would result in the following appearing in your book (without the yellow highlight!):