In The Private Patient (Knopf  2008,  $25.95) P.D. James, who began her celebrated and prolific mystery writing career in her forties, has delivered an excellent Gothic tangle of a mystery. Her protagonist, investigative journalist Rhoda Gradwyn, has  lived throughout her life and career with a brutally disfiguring facial scar. Shortly after her 47th birthday, she checks into an exclusive private…