Insert this function into your functions file or plugin in order to filter your post type archive results.

filterArchive.php

<?php
function my_new_posts_filter($query) {
    // in place of "press" put your custom post type -- you can add additional post types by duplicating the if block.
    // you can also add more query variables such as 'order'. This is extremely useful
    // the is_main_query makes sure you don't indiscriminantly filter secondary loops as well.
	if ( !is_admin() && is_post_type_archive('book') && $query->is_main_query() ) {
		 $query->set( 'posts_per_page', '20' );
	}
}

add_action('pre_get_posts','my_new_posts_filter');
?>