Ted Conover, the intrepid author of Coyotes, about the world of illegal Mexican immigrants, spent a year as a prison guard at Sing Sing. Newjack, his account of that experience, is a milestone in American journalism: a book that casts new and unexpected light on this nation's prison crisis and sets a new standard for courageous, in-depth reporting.         At the infamous Sing Sing, once a model prison but now New York State's most troubled maximum-security facility, Conover goes to work as a gallery officer, working shifts in which he alone must supervise scores of violent inner-city felons. He soon learns the impossibility of doing his job by the book. What should he do when he feels the hair-raising tingle that tells him a fight is about to break out? When he loses a key in a tussle? When a prisoner punches him in the head? Little by little, he learns to walk the fine line between leniency and tyranny that distinguishes a good guard.        Along the way, we meet a cast of characters that includes a tough but appealing supervisor named Mama Cradle; a range of mentally ill prisoners, or "bugs"; some of the jail's more flamboyant transvestites; and a philosophical, charismatic inmate who points out to Conover that the United States is building new prisons for future felons who are now only four and five years old. Conover also gives us a history of Sing Sing (it was built by inmates, and for decades was the nation's capital of capital punishment) in a chapter that serves as a brilliant short course in America's penal system.        With empathy and insight, Newjack tells the story of a harsh, hidden world and dramatizes the conflict between the necessity to isolate criminals and the dehumanization—of guards as well as inmates—that almost inevitably takes place behind bars.