book.settings.yml

book_admin_settings in drupal/core/modules/book/book.admin.inc

Form constructor for the book settings form.

book_node_type_update in drupal/core/modules/book/book.module

Implements hook_node_type_update().

book_update_8000 in drupal/core/modules/book/book.install

Move book settings from variables to config.

allowed_types:
  - book
block:
  navigation:
    mode: all pages
child_type: book