On the eve of the Civil War, the estimated value of the U.S. enslaved population exceeded $3 billion -- triple that of investments nationwide in factories, railroads, and banks combined, and worth more even than the South's lucrative farmland. Not only an object to be traded and used, the slave was also a kind of currency, a form of value that anchored the market itself. And this value was not destroyed in the war. Slavery still structured social...