On today's ST, which originally aired earlier this year, we speak with the journalist, author, and veteran urbanologist Alan Ehrenhalt, whose books include "The United States of Ambition" and "The Lost City." Ehrenhalt's latest book, "The Great Inversion and the Future of the American City," is now out in paperback. This book argues that the roles and characteristics of America's cities and suburbs are basically changing places with one another --- younger adults and affluent retirees are moving into the city, while immigrants and the less affluent are moving out to the 'burbs. As one critic for Booklist noted of "The Great Inversion and the Future of the American City" when it was first published: "In the future, American cities could look like late-nineteenth-century Vienna, with lively, affluent metropolitan core areas and the lower classes consigned to life in peripheral suburbs. Such cities will go well beyond gentrification and involve the displacement of the poor in inner-city areas by the wealthy, according to urbanologist Ehrenhalt. He details how the trend toward such cities is already apparent in Chicago, Atlanta, Washington, Houston, and other metropolitan areas. Drawing on census data and economic research, he examines the factors behind the trend, including mass transit, retail and housing development in downtown locations, and the declining appeal of long commutes to distant suburbs. Ehrenhalt also offers detailed portraits of the future of suburban sprawl in areas struggling to re-create the appeal of cities by developing more accessible commercial zones. This is an engaging look at demographic changes that promise a very different future for cities and suburbs."