In the settings page of your book, you can enable or disable discussions. People who have a read access to your book will be able to start discussions on your book from the discussions tab of your book's dashboard.
If you allowed discussions, you can also allow inline comments in the settings of your book. If you just enabled this feature, it will only be effective the next time you update the content of your book. Then the website for your book will display a "+" button near paragraphs for people to start discussions from there.