Summarize the book Newjack: Guarding Sing Sing.

In Newjack: Guarding Sing Sing, a journalist decides to work as a prison guard in a New York prison so that he can write about his experiences and the life of the inmates there. He finds that he's changed by the experience.
Ted Conover gets a job at Sing Sing after being denied access to write about it through standard channels. He goes through the application process and training and then gets into the thick of it in the real job.
He starts the process at the Albany Training Academy, where Sing Sing guards learn to handle the demands of the tough job. There, he makes connections and builds relationships with other correctional officers. He also realizes how difficult the job will be because the training is grueling and physically demanding.
When he actually arrives at Sing Sing, he finds that the job is as hard emotionally as it is physically. The days feel long, and he gets little respect. He doesn't enjoy the way most of the other officers do their jobs. The one person he does like and tries to emulate is a correctional officer who treats the inmates with respect and is willing to joke around with them. He clearly sees them as more than their crimes.
Conover works on the floor, in the gym, in the psych unit, and, at times, in the place where inmates are sent for solitary confinement. He has to deal with finding contraband items and deciding when he should push the rules and when he should relax. It's a balance between keeping the inmates settled and maintaining his authority as an officer.
Conover also tells some of the stories of the men who are imprisoned at Sing Sing. They're often tragic and show intelligent or mentally disturbed people who aren't getting the care, treatment, or stimulation they need to improve their lives. At the same time, though, some are dangerous, and many inmates and officers are hurt in altercations during Conover's time at Sing Sing.

Early in journalist Ted Conover's nonfiction account of the year he spent as a corrections officer in New York State's Sing Sing prison, Newjack: Guarding Sing Sing, he meticulously describes the prison's decrepit condition, noting that "If the whole structure were radically shrunk, the uninitiated might perceive a vaguely agricultural purpose; the cages might be thought to contain chickens, or mink." Conover's point is that the American criminal justice system has failed miserably and that the conditions in which the nation's prison population is housed and treated represents a serious failure to understand the nature of crime and punishment; it also represents a collective failure to adopt a penal system consistent with the country's principles. The prisoners he helped guard were housed like animals, and it should, he concludes, come to little surprise that they responded accordingly.
Newjack is not only a depiction of life inside Sing Sing. It is also a history of the prison itself, from its initial construction during the 1820s to the present. It also serves as a critical analysis of America's criminal justice system. One of the topics covered by Conover, in addition to descriptions of his responsibilities as a prison guard with an emphasis on both the mundane nature of the job and the terror that is inherent in having to confront dangerous, violent convicts daily, is the long unsuccessful history of implementing prison reform. 
Thomas Mott Osborne was warden of Sing Sing in the early part of the 20th century. Osborne had—like Conover a century later and for a much briefer time—checked himself into a prison with his identity concealed so that he could see for himself the conditions in which the United States housed its inmates. Passing himself off as a convict, Osborne was appalled by what he witnessed and was determined to reform the correctional system in New York State. He failed and Conover's observations during his time as a prison guard mirrored those of Osborne. This further illuminates the inability or unwillingness of the United States to adopt measures to reduce the prison population (like many others, Conover is critical of mandatory sentence guidelines and the excessive use of incarceration for nonviolent drug offenses) and to address the problem of recidivism. 
Conover’s examination of the criminal justice system and firsthand observations of life inside a particularly notorious penal institution was intended to illuminate the shocking contradictions between American values and the way it treats over one million of its citizens. The United States, he declares, has failed to create a criminal justice system that rehabilitates those it incarcerates and it does nothing to make our cities safer.