I have recently finished reading what may be the most important book of the decade on the contemporary economy. It is not Thomas Piketty’s controversial “Capital in the Twenty-First Century” or Robert Gordon’s magisterial “Rise and Fall of American Growth.” It is Berkeley economist Enrico Moretti’s short, lucid, nontechnical volume, “The New Geography of Jobs,” published in 2012.

Mr. Moretti’s book offers a compelling and simple explanation of the most fundamental economic trend of our time—the widening split between dynamic...