Custom Post not Deleting on Uninstall

try with this :

$args = array (
    'post_type' => 'book',
    'nopaging' => true
  );
  $myquery = new WP_Query ($args);
  while ($myquery->have_posts ()) {
    $myquery->the_post ();
    $id = get_the_ID ();
    wp_delete_post ($id, true);
  }
  wp_reset_postdata ();

let me know if this works for you!