Fake mark a menu item as current_page_item/active in WordPress

You can add a custom class via the nav_menu_css_class filter, like so:

function wpa_65178_nav_class( $classes, $item )
{
    if( is_singular('book') && $item->title == 'Products & Services' )
    {
        $classes[] = 'current_page_ancestor';
    }
    return $classes;
}
add_filter( 'nav_menu_css_class', 'wpa_65178_nav_class', 10, 2 );