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 );