如果我理解正确的话，这本书和评论有着相同的分类法。

那么，只需在single-books.php, 将其保存在变量中，然后使用该变量为\'tax_query\':

if ( have_posts() ) {
  the_post();

  // get the term for the current book and save in a variable
  $book_terms = get_the_terms( get_the_ID(), \'your_custom_taxonomy\' );
  $book_term = ! empty( $book_terms ) ? array_shift( $book_terms ) : FALSE;

  // loop code for the book goes here

}

if ( $book_term ) { // if we get the term for current book...
  // second query
  $args = array(
    \'post_type\' => \'review\',
    \'tax_query\' => array(
       array(
         \'taxonomy\' => \'your_custom_taxonomy\',
         \'terms\' => array( $book_term->term_id )
       )
     )
  );
  $reviews = new WP_Query( $args );

  // loop code for review goes here
}