This is an interesting module. It seems like a method that assists with the navigation of pages on a web site. I was hoping to use it to supply a hierarchy of pages in the Content view of Drupal.
Here is a link to the documentation → https://www.drupal.org/docs/8/core/modules/book/overview
Here is link to the sample I setup to try it → https://store.technologynursery.org/node/2
Here is what I entered for pages to try it out.
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Jims TOP Book Page ← Book type page
--Jims 1 ← Book type page
----Jims 1.1 ← Article type page
----Jims 1.2 ← Article type page
--Jims 2 ← Book type page
----Jims 2.1 ← Basic type page
----Jims 2.2 ← Basic type page