BookOutlineForm::form

public BookOutlineForm::form(array $form, FormStateInterface $form_state)

public function form(array $form, FormStateInterface $form_state) {
  $form['#title'] = $this->entity->label();

  if (!isset($this->entity->book)) {
    // The node is not part of any book yet - set default options.
    $this->entity->book = $this->bookManager->getLinkDefaults($this->entity->id());
  }
  else {
    $this->entity->book['original_bid'] = $this->entity->book['bid'];
  }

  // Find the depth limit for the parent select.
  if (!isset($this->entity->book['parent_depth_limit'])) {
    $this->entity->book['parent_depth_limit'] = $this->bookManager->getParentDepthLimit($this->entity->book);
  }
  $form = $this->bookManager->addFormElements($form, $form_state, $this->entity, $this->currentUser(), FALSE);

  return $form;
}