Agatha Christie’s autobiography, published posthumously in 1977, provides a fascinating window into the economic life of middle-class Britons a century ago. The year was 1919, the Great War had just ended, and Christie’s husband Archie had just been demobilized as an officer in the British military. The couple’s annual income was around around £700 ($50,000 in today’s dollars)—£500 ($36,000) from