Mostrar campo personalizado y descargar libro desde página similar a single.php

add_filter( 'rewrite_rules_array','my_insert_rewrite_rules' );
add_filter( 'query_vars','my_insert_query_vars' );
add_action( 'wp_loaded','my_flush_rules' );

function my_flush_rules(){
	$rules = get_option( 'rewrite_rules' );

	if ( ! isset( $rules['descargar/(.*)?$'] ) ) {
		global $wp_rewrite;
	   	$wp_rewrite->flush_rules();
	}
}

function my_insert_rewrite_rules( $rules )
{
	$newrules = array();
	$newrules['descargar/(.*)?$'] = 'index.php?pagename=descargar&id=$matches[1]';
	return $newrules + $rules;
}

function my_insert_query_vars( $vars )
{
    array_push($vars, 'id');
    return $vars;
}

$args=array(
  'name' => get_query_var('id'),
  'post_type' => 'post',
  'post_status' => 'publish',
  'showposts' => 1,
  'caller_get_posts'=> 1
);
$my_posts = get_posts($args);
echo get_post_meta($my_posts[0]->ID, 'nombre_del_campo', true);