The days of hard cash may be numbered. Having invented paper money more than 1,000 years ago, China is now leading the charge into credit-card free online payments. It is also developing its own digital currency, one that could allow the state to observe and control every financial transaction. Martin Chorzempa’s “The Cashless Revolution” engagingly describes this state of affairs and, along the way, presents a cautionary tale, since dozens of countries around the world are contemplating the launch of their own digital currencies. Mr. Chorzempa, a fellow at the Peterson Institute for International Economics, explains how the People’s Republic...