CARLO ROVELLI is the man who can spin hard physics into pure gold. The Order of Time is his third book. Like the first (Seven Brief Lessons on Physics), it has been an instant bestseller. In this state-of-the-art survey of what physicists thought and now think about the nature of time, Rovelli is both unsettling (time does not exist) and philosophical (the study of time "does nothing but return us to ourselves"). IT MAY not be a classic Christmas whodunnit, but The Beautiful Cure is a page-turner.