drupal/core/modules/book/book.libraries.yml

drupal.book:
  version: VERSION
  js:
    book.js: {}
  dependencies:
    - core/jquery
    - core/drupal
    - core/drupal.form

navigation:
  version: VERSION
  css:
    theme:
      css/book.theme.css: {}

admin:
  version: VERSION
  css:
    theme:
      css/book.admin.css: {}