Build a custom meta query

custom-book-author-query.php

<?php
/**
 * Build a custom query
 *
 * @param $query obj The WP_Query instance (passed by reference)
 *
 * @link https://codex.wordpress.org/Plugin_API/Action_Reference/pre_get_posts
 */
function myplugin_pre_get_posts( $query ) {
	// check if the user is requesting an admin page 
	// or current query is not the main query
	if ( is_admin() || ! $query->is_main_query() ){
		return;
	}

	// edit the query only when post type is 'food'
	// if it isn't, return
	if ( !is_post_type_archive( 'book' ) ){
		return;
	}

	$meta_query = array();

	// add meta_query elements
	if( !empty( get_query_var( 'book-author' ) ) ){
		$meta_query[] = array( 'key' => 'author_surname', 'value' => get_query_var( 'book-author' ), 'compare' => 'LIKE' );
	}

	if( count( $meta_query ) > 1 ){
		$meta_query['relation'] = 'AND';
	}

	if( count( $meta_query ) > 0 ){
		$query->set( 'meta_query', $meta_query );
	}
}
add_action( 'pre_get_posts', 'myplugin_pre_get_posts', 1 );