Update to Drupal 8.0.0 beta 14. For more information, see https://drupal.org/node/2544542

core/modules/book/src/Plugin/migrate/destination/Book.php

@@ -0,0 +1,36 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\book\Plugin\migrate\destination\Book.
+ */
+
+namespace Drupal\book\Plugin\migrate\destination;
+
+use Drupal\Core\Entity\EntityInterface;
+use Drupal\migrate\Plugin\migrate\destination\EntityContentBase;
+use Drupal\migrate\Row;
+
+/**
+ * @MigrateDestination(
+ *   id = "book",
+ *   provider = "book"
+ * )
+ */
+class Book extends EntityContentBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static function getEntityTypeId($plugin_id) {
+    return 'node';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function updateEntity(EntityInterface $entity, Row $row) {
+    $entity->book = $row->getDestinationProperty('book');
+  }
+
+}

core/modules/book/src/Plugin/migrate/source/d6/Book.php

@@ -0,0 +1,68 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\book\Plugin\migrate\source\d6\Book.
+ */
+
+namespace Drupal\book\Plugin\migrate\source\d6;
+
+use Drupal\migrate_drupal\Plugin\migrate\source\DrupalSqlBase;
+
+/**
+ * Drupal 6 book source.
+ *
+ * @MigrateSource(
+ *   id = "d6_book"
+ * )
+ */
+class Book extends DrupalSqlBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function query() {
+    $query = $this->select('book', 'b')->fields('b', array('nid', 'bid'));
+    $query->join('menu_links', 'ml', 'b.mlid = ml.mlid');
+    $ml_fields = array('mlid', 'plid', 'weight', 'has_children', 'depth');
+    for ($i = 1; $i <= 9; $i++) {
+      $field = "p$i";
+      $ml_fields[] = $field;
+      $query->orderBy($field);
+    }
+    $query->fields('ml', $ml_fields);
+    return $query;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getIds() {
+    $ids['mlid']['type'] = 'integer';
+    $ids['mlid']['alias'] = 'ml';
+    return $ids;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function fields() {
+    return array(
+      'nid' => $this->t('Node ID'),
+      'bid' => $this->t('Book ID'),
+      'mlid' => $this->t('Menu link ID'),
+      'plid' => $this->t('Parent link ID'),
+      'weight' => $this->t('Weight'),
+      'p1' => $this->t('The first mlid in the materialized path.'),
+      'p2' => $this->t('The second mlid in the materialized path.'),
+      'p3' => $this->t('The third mlid in the materialized path.'),
+      'p4' => $this->t('The fourth mlid in the materialized path.'),
+      'p5' => $this->t('The fifth mlid in the materialized path.'),
+      'p6' => $this->t('The sixth mlid in the materialized path.'),
+      'p7' => $this->t('The seventh mlid in the materialized path.'),
+      'p8' => $this->t('The eight mlid in the materialized path.'),
+      'p9' => $this->t('The nine mlid in the materialized path.'),
+    );
+  }
+
+}