Run next query based on first query’s term

If I understand correctly, the book and the reviews share same taxonomy.

Then is just a matter of get the taxonomy term for the book in the single-books.php, save it in a variable and then run a query using that variable for the '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
}