Book.php

<?php

namespace Drupal\book\Plugin\migrate\source\d6;

use Drupal\book\Plugin\migrate\source\Book as BookGeneral;
@trigger_error('Book is deprecated in Drupal 8.6.x and will be removed before Drupal 9.0.x. Use \\Drupal\\book\\Plugin\\migrate\\source\\Book instead. See https://www.drupal.org/node/2947487 for more information.', E_USER_DEPRECATED);

/**
 * Drupal 6 book source.
 *
 * @MigrateSource(
 *   id = "d6_book",
 *   source_module = "book"
 * )
 *
 * @deprecated in drupal:8.6.0 and is removed from drupal:9.0.0. Use
 * \Drupal\book\Plugin\migrate\source\Book instead. See
 * https://www.drupal.org/node/2947487 for more information.
 */
class Book extends BookGeneral {

}