Book.php

<?php

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 $row) {
        if ($entity->book) {
            $book = $row->getDestinationProperty('book');
            foreach ($book as $key => $value) {
                $entity->book[$key] = $value;
            }
        }
        else {
            $entity->book = $row->getDestinationProperty('book');
        }
        return parent::updateEntity($entity, $row);
    }

}