book-2.0.x-dev/book.libraries.yml

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

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

book-edit-form:
  version: VERSION
  css:
    theme:
      css/book-edit-form.css: {}