Form constructor for administering a single book's hierarchy.
Drupal\node\Node $node: The node of the top-level page in the book.
function book_admin_edit($form, $form_state, Node $node) {
  drupal_set_title($node
    ->label());
  $form['#node'] = $node;
  _book_admin_table($node, $form);
  $form['save'] = array(
    '#type' => 'submit',
    '#value' => t('Save book pages'),
  );
  return $form;
}