How do I require authorization / login to view a specific set of posts / pages?

This is just a modification of timshutes’ answer – if you want specific pages to require login and don’t want to put them into a custom post type, you can add to functions.php:

add_shortcode('need_login', 'shortcode_needLogin');

function shortcode_needLogin() {
    if (!is_user_logged_in()) {
        auth_redirect();
    }
}

And then at the top of the pages you want to require login for, you can just do:

[need_login]