The author of the authoritative three-volume biography of John Maynard Keynes provides here a welcome short introduction to Keynesian economics, which he argues is still relevant to the modern U.S. economy. Keynes specified three important contentions. First, uncertainty needs to be taken seriously in modern economics, and this fundamental uncertainty -- the unknown unknowns -- cannot be adequately captured with the statistical techniques fashionable in the study of economics. Second, once dislodged from a satisfactory state, modern economies cannot be relied on to return smoothly and automatically to that state -- at least not quickly enough to be politically tolerable. Third, civilization cannot thrive if efficiency and moneymaking are held as its highest values. The book offers clear and cogent critiques of modern macroeconomic thought, along with a brief but useful summary of what went wrong in 2007-9.