In Drupal 8, for the Book module, you can plug any content type page(e.g. Article, Basic page, etc) into any book. The child page doesn't need to be a Book page.

Add node page to book

To add a node page in any book, simply open the node page in edit mode and in the BOOK OUTLINE section, select in which book you would like to plug in.

Book outline permission

Please note that your user need to have the Administer book outlines permission for the BOOK OUTLINE section to appear. You can add the permission from People > Permissions tab.