Adding a book does not add content to it. This is a second step where Chapters (essentially pages) and sub-chapters are added.
Click on the Book name to open the Edit Chapter Screen where you can add content.
Chapter Title: Type your Chapter title. This will appear in the ToC.
Content: Type your page content in here.
As well as adding text, you can also add images, tables and hyperlinks as you can in a page.
Once complete Save changes
The page displays and the Table of Contents (ToC) block is turned on.
Navigation arrows are automatically added to the top and bottom of book pages.
Use the ToC editing icons to:
Use the + icon to add additional pages.
Once there is more than one page, a "Move" icon becomes available for reordering individual chapters / pages.
When Editing a Chapter, tick the Subchapter check box to make it a sub-page of the previous Chapter.
See Chapter and Sub-chapter ToC displays with editing on and off respectively.