This is a very upsetting book. The Harvard sociologist Matthew Desmond spent a year and a half living in low-income housing in Milwaukee — first in a trailer park on the mostly white South Side and later in a rooming house in the black inner city. Desmond himself plays no part in the body of the story, but he reports what he saw and heard, using a digital recorder and filling in the rest with double-sourced eyewitness accounts and official documents. And what he saw was upsetting, though not always, or not particularly, in a dramatic way. The things that happened to the people he lived among happened too often and routinely to seem extraordinary to them. For the two or three weeks I was reading the book, it formed my topic of conversation with friends, and at night, when I went to sleep, it filled my thoughts.