On Drupal, the node title is always required. On books, the title is even more important, since it is used for the links to the other pages of the book, shown after the page content.
I took the screenshot on Drupal 7, but the same is true for Drupal 8.
The only way to avoid entering a title would be having a module that automatically adds a title to nodes, like the Automatic Nodetitles module, or the Automatic Entity Label (which has a version for Drupal 7 and Drupal 8).
Automatic Nodetitles is a small and efficient module that allows hiding of the content title field in the form. To prevent empty content title fields one can configure it to generate the title by a given pattern.
"Automatic Entity Label" is a small and efficient module that allows hiding of entity label fields. To prevent empty labels it can be configured to generate the label automatically by a given pattern.