A body in the back of a taxi begins an elegantly constructed mystery, perhaps the finest of Marsh’s 1930s novels.The season had begun. Débutantes and chaperones were planning their luncheons, teas, dinners, balls. And the blackmailer was planning his strategies, stalking his next victim.But Chief Detective Inspector Roderick Alleyn knew that something was up. He had already planted his friend Lord Robert Gospell at the scene.But someone else got there first…