“Mistress of the Ritz” by Melanie Benjamin(“The Aviator’s Wife”) is a “captivating novel” that is based on an American woman who secretly worked for the French Resistance during World War II. By day, she plays  hostess to the occupying Germans at the iconic Hotel Ritz in Paris; by night she assists the resistance.  Cameo’s by Coco Chanel and Ernest Hemingway add to the period flavor.   And, as in “A Gentleman in Moscow”, the hotel is part of the story.