Add book-author rewrite rule

add-rewrite-rule.php

<?php
/**
 * Add rewrite rules
 *
 * @link https://codex.wordpress.org/Rewrite_API/add_rewrite_rule
 */
function myplugin_rewrite_rule() {
	add_rewrite_rule( '^book/book-author/([^/]*)/?', 'index.php?post_type=book&book-author=$matches[1]','top' );
}
add_action('init', 'myplugin_rewrite_rule', 10, 0);