Yeap, you’re right. This does the trick. Thanks!
Just for future reference, here is my working code:
add_action( 'pre_get_posts', 'book_custom_orderby' );
function book_custom_orderby( $query ) {
if ( ! is_admin() )
return;
if (empty($_GET['orderby'])) {
$query->set( 'meta_key', 'order-no' );
$query->set( 'orderby', 'meta_value_num' );
$query->set( 'order', 'ASC' );
}
}