In awarding a National Humanities Medal to photographer Camilo José Vergara last year, President Obama praised his studies of American cities: “By capturing images of urban settings over time, his sequences reflect the vibrant culture of our changing communities and document the enduring spirit that shines through decay.” The president’s words are an apt description of Vergara’s new book “Harlem: The Unmaking of a Ghetto” (University of Chicago). When Vergara began photographing Harlem in 1970, he expected to document its collapse. Yet the community has survived. Vergara’s commentary is a mix of history, sociology, and memoir. He captures telling personal details as well as the long arc of change and rebirth.