How do I add a section of my own to my book, such as a Special Offer, a Bonus or a Reading Group Guide?

From version 1.9.24, open the book's page. Under the front matter in the chapter list, press "+ Add a front matter section", or after the chapters press "+ Add a back matter section". Type its heading, for example "Special Offer" or "Bonus", and press "Add". The editor opens on the new section: write or paste its text, which is saved as you type. The section is printed in every export with your heading, where you placed it, and its row is marked "yours".