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

PHP Code:
<?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?

That's WordPress of course.

Many thanks!