class TopLevelBook

class TopLevelBook extends Node {
    
    /**
     * The node storage controller.
     *
     * @var \Drupal\node\NodeStorageInterface
     */
    protected $nodeStorage;
    
    /**
     * Constructs a Drupal\book\Plugin\views\argument_default\TopLevelBook object.
     *
     * @param array $configuration
     *   A configuration array containing information about the plugin instance.
     * @param string $plugin_id
     *   The plugin_id for the plugin instance.
     * @param array $plugin_definition
     *   The plugin implementation definition.
     * @param \Drupal\Core\Routing\RouteMatchInterface $route_match
     *   The route match.
     * @param \Drupal\node\NodeStorageInterface $node_storage
     *   The node storage controller.
     */
    public function __construct(array $configuration, $plugin_id, array $plugin_definition, RouteMatchInterface $route_match, NodeStorageInterface $node_storage) {
        parent::__construct($configuration, $plugin_id, $plugin_definition, $route_match);
        $this->nodeStorage = $node_storage;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
        return new static($configuration, $plugin_id, $plugin_definition, $container->get('current_route_match'), $container->get('entity_type.manager')
            ->getStorage('node'));
    }
    
    /**
     * {@inheritdoc}
     */
    public function getArgument() {
        // Use the argument_default_node plugin to get the nid argument.
        $nid = parent::getArgument();
        if (!empty($nid)) {
            $node = $this->nodeStorage
                ->load($nid);
            if (isset($node->book['bid'])) {
                return $node->book['bid'];
            }
        }
    }

}