How to get related posts by tags?

Hi,
This is generating general posts for me and I add them in a box on the sidebar.

<?php

			$args = array(
			'post_type' => 'books',
			'posts_per_page' => 6,
			'orderby'=>'date',
			'order'=>DESC,); 	

			query_posts( $args );
			while ( have_posts() ) {
			the_post();

			?>

Do you know what do I need to add in order to get the related posts by tags and not just general ones?

Many thanks!