America's gross domestic product is about $17 trillion. But what does this number mean? A new short book (GDP: A Brief but Affectionate History by Diana Coyle, 2014) explains where this concept came from, what actually gets measured and what's missing, and how to make it better. A nice easy read for anyone wondering why government measures of economic progress don't count environmental damage and undercount the digital economy. (See earlier related posts on measuring environmental performance February 4; forecasting January 15; the tragedy of bees December 28, 2013; Gross Domestic Freebie December 4, 2013.)