(CNN) The first book John Bunn fell in love with, curled up in his cell at a maximum-security prison in upstate New York, was Sister Souljah's novel "The Coldest Winter Ever."

In the book, a maternal woman advocates for the improvement of her black community in Brooklyn as she watches the people she loves suffer from the consequences of incarceration, violence and a seemingly endless cycle of poverty.

"I related to that book on so many levels," Bunn says.

John Bunn outside the Kingsborough Projects in Crown Heights, Brooklyn, where Robert Crosson and Rolando Neischer were carjacked and shot in August of 1991.

Bunn knows more than most what it's like to face injustice. Arrested and imprisoned as an adolescent in New York City, he spent 17 years in jail for a crime he didn't commit and a further decade on parole, fighting for his exoneration. In that time, he battled, among others, the courts, police investigators, PTSD and the challenges of illiteracy. He was 16 before he could read and write.

Today Bunn is 41 and a free man at last, mentoring at-risk young people and advocating for the power of reading through his own program that brings books to prisons.

In many ways, his own story sounds straight out of a Sister Souljah book. Except that Bunn, who survived years of wrongful incarceration with his humanity intact, is determined to write the next chapter himself.

The arrest

Bunn's ordeal began on August 14, 1991, when he was sitting in the kitchen of his mother's apartment in Crown Heights, Brooklyn.

It was 90 degrees in the shade and the AC was broken. Outside he could hear hip-hop music playing from passing cars and the thwack of basketballs on pavement as kids made their way to the courts. Bunn's mother, Maureen, was making pancakes, his two-year-old sister, India, cooing in her high chair.

Bunn, 14 years old and out of school for the summer, was ready for a typical day of playing ball and demonstrating his famous back flips in and around the four-block radius between the apartment on Ralph Street (his mom) and the house on St Marks (his grandma). Those four blocks, snug between the love of the two women who raised him, were his whole world.

John Bunn, 14, at home in Crown Heights, Brooklyn.

But then, a bang on the door. It was the police.

"They wanted to take me down to the police station for questioning," Bunn recalls now, sitting in that same small apartment festooned with family photos, nearly three decades later.

He was taken to Brooklyn's 77th precinct, put in a room and handcuffed to a pole.

"The interrogation was led by a detective by the name of Louis Scarcella. And he was threatening me, telling me that I was never coming home if I wouldn't tell him what he wanted to know. He also told me that they already had beat up my co-defendant, that they had slammed his head into a wall and they already had him," he recalls.

The co-defendant? A 17-year-old Brooklyn boy named Rosean Hargrave. Bunn knew Hargrave "from the block," although he and the older boy were never more than acquaintances. But, as he soon found out, they were both now suspected of the same crime: The killing of an off-duty Rikers Island corrections officer named Rolando Neischer.

"I kept telling them, "No, I didn't have any knowledge of it," Bunn recalls.

John Bunn, 14, right, and Rosean Hargrove, 17, arrested at the 77th Precinct in Brooklyn in August 1991. NYPD Detective Louis Scarcella (open jacket and loose tie) holds Bunn.

But Detective Scarcella, who worked in the Brooklyn North homicide unit for years before retiring in 1999, told the young John he did not believe him.

Bunn's eyes fill with tears as he describes the moment he was placed in a police lineup with "grown men." As an adult, Bunn, a slight man with a gentle disposition and a shy smile, stands only 5 feet 6 inches. At 14, he estimates he was no taller than 5-foot-2. He was so much smaller than the adults he was lined up with that the detectives had to improvise.

They brought in stools so the lineup could be done sitting down. Bunn did what he was told. He sat down and held up a number.

A couple of minutes later, Scarcella came back into the room.

"He told me, 'It was my lucky day,' that I got picked," Bunn says, grimacing.

"Ever since then, I've been fighting to prove my innocence," he says, wiping his face and adjusting his hat.

On the front of his baseball cap, in bold white letters, are the words, "WRONGFULLY CONVICTED. On the side, "VICTIMS OF DETECTIVE LOUIS SCARCELLA."

The hat, he adds, "speaks for itself."

The night of the killing

The day before Bunn was arrested, two off-duty Rikers Island correction officers and old friends, Rolando Neischer and Robert Crosson, were sitting in a car, chatting outside the Kingsborough projects, a housing project in Crown Heights. It was about 4 a.m.

Suddenly, two men on bicycles approached the car, pointed guns and ordered the pair out of their vehicle. Crosson later described at trial how a gun then "went off," the bullet ripping through his hand. He managed to get out of the car and sprinted away, losing track of his friend.

At some point, Neischer had grabbed his own firearm and fought back.

In the ensuing gunfight, before the attackers jumped in the car and drove away, Neischer was shot five times. He was found slumped against a fence, bleeding, but still alive, 192 feet from where the car had been parked.

The next day, they came for John Bunn, with Crosson identifying both him and Hargrave in police lineups. Three days after that, Neischer died from his wounds.

On the night Neischer and Crosson were carjacked, Bunn had no inkling of what was to come. But his mother heard a portent.

At about 4:30 a.m., Maureen Bunn awoke from a deep sleep to the sound of gunshots. They sounded like they were coming from the backyard of her housing complex, near Kingsborough.

"I thought, oh my god, I hope everybody's ok," she says, recalling the incident 27 years later.

Instinctively, she padded to her children's room, peering into the dark to check on them. Minutes after the bullets that killed Neischer tracked their fatal course through the soupy August air that night, Maureen noted with relief that her kids were safe.

John lay sound asleep in his bunk.

The trial

On August 17, Bunn was formally charged with robbery and murder. He was sent to the notorious Spofford Juvenile Detention Center in the Bronx, a vermin-infested concrete building that was shut down in 2011 after allegations of violence and abuse. There, he waited for 16 months.

Spofford was "one of the most violent places I ever experienced," Bunn says. "The staff members were very abusive. It was like gladiator school. You had to fight maybe three or four times a day."

What's more, there was "a target" on his back because he had been charged with killing a corrections officer. The months rolled by as he waited for trial. His 15th birthday came and went. An increasingly depressed and scared Bunn didn't tell anyone about it.

A 2005 photo of the Bridges Juvenile Center, where John Bunn spent nearly a year and a half awaiting trial.

"Things like birthdays stopped existing to me," he says. "Time stopped in there."

Still, Bunn and his mother believed the trial would show he was not Neischer's killer. Among other factors, Crosson, the sole witness to the crime, had described the carjackers as being light-skinned men in their 20s's. John was darker skinned and a short, slight-framed teenager.

Bunn and Hargrave's trial took place in November of 1992, on Thanksgiving eve. Detective Scarcella had testified in pre-trial hearings, but on the day of the trial itself, just four people took the stand: the first police officer to arrive at the scene, the medical examiner, a 77th precinct detective, and Crosson, the sole primary witness who had picked both boys out of lineups.

The trial lasted just one day. Bunn and Hargrave were both found guilty.

Brooklyn detectives Louis N. Scarcella, left, and Stephen W. Chmil at the Brooklyn D.A. office on April 19, 1994.

"The truth didn't prevail, it didn't come out ... My side of the story never came out, either. I never got a chance to have a voice. I never got a chance to say anything," Bunn says.

Bunn was sentenced to 20 years to life, though it was later reduced to nine years to life after lawyers successfully argued that he had been illegally charged as an adult. Hargrave received a sentence of 30 years to life.

When, Maureen Bunn heard the word "life" she had to be rushed to the hospital.

"They said I had a mild heart attack. I couldn't breathe. I don't even remember when the ambulance came. I went into another world. It was just disbelief," she says.

Both Bunn and Hargrave filed appeals which were denied.

Imprisonment

After being found guilty. Bunn was immediately taken to a youth facility upstate. Maureen visited as much as she could, spending all her money on long bus rides upstate as the household bills and eviction notices stacked up.

But John was isolated physically -- and mentally. For a long time, he had hidden a secret. He had never learned to read or write.

"I was further away from my family. I wanted to communicate with my mother, because I felt like me and her didn't have the best relationship. And I was out there, and I just wanted to tell her how I felt, and that motivated me," he says.

Determined to be able to write letters to his mom, Bunn started with dictionaries and children's books, working with teachers on how to sound words out, letter by letter. It was humbling at first, but he learned fast. And as he got the hang of it, he says, "it did something for my self-esteem, and my imagination."

By the time he was 17 Bunn had his GED, and was reading anything he could get his hands on from the prison library. But turning 17 also marked a more sobering milestone: He would soon be eligible to be transferred to an adult facility.

Photos of John Bunn during his time in jail line the walls at the home of his mother, Maureen.

"I got my GED and I graduated to state prison," says Bunn, adding that he remembers the bus journey to the adult facility as "one of the scariest feelings I ever experienced."

"You hear all of the stories about how people get raped, and there's knives, and there's razors. But all along that bus ride, I went from being fearful and intimidated, to suddenly me becoming aggressive, because I felt like I had to be aggressive in order for them not to take advantage of me," he says.

Bunn noticed that, increasingly, he was finding his anger hard to control.

"I became institutionalized to the point where I start letting that experience make me angry. I started being bitter in there and started getting into violent situations."

The Elmira Correctional Facility in Elmira, New York, where John did much of his reading.

Through it all, one thing kept him level-headed. To those in the prison, he may have been John Bunn, inmate number 7129748Q, Brooklyn boy, corrections officer killer. But, with a book in his hand, lying in his bunk on G block at Elmira Correctional Facility, an imposing Victorian building that sits high on a hill in upstate New York, Bunn felt free.

In the daytime, he began taking anger management classes, passing them with such flying colors that he became a qualified anger management counselor himself, running his own program for inmates.

At night, he would return to his cell where he would read slowly so he had "something to look forward to," savoring each word amidst the familiar soundtrack of the prison at night -- the hum of 50 radios all softly playing different stations, the monologues of top-dog inmates holding court for 10 minutes at a time as others kept a respectful silence, and the ever-present clang of the gates that slammed and opened, slammed and opened, signaling the night and heralding the morning.

John, his mother Maureen and sister India during a family visit at Elmira.

"I wrote my mother one day ... and I said, "They can lock my body, but they can't trap my mind," Bunn says.

"The power of reading made me feel that way. I felt trapped without a voice for so long, but the power of reading could take my imagination, and take me to anywhere in this universe."

In 2006, Bunn saved a prison counselor from being violently assaulted and raped by another inmate. His heroic actions were a factor in the parole board's decision to release him that year.

Bunn walked out of Elmira, wearing a shirt, shorts and a pair of Timberland boots bought for him by his family, and with his $40 of "gate money," earned from his $7.50 a week prison salary, in his pocket.

"When I got in the lobby, my brother and cousin were there. Once I got to them I felt safe," he says. "It was the first time I felt like I was safe again in 17 years."

Exoneration

But out on parole, Bunn struggled. With his criminal record as a convicted murderer, he couldn't get a job. He was diagnosed with PTSD and was granted social security and disability.

Then, in 2008, after struggling with depression, he failed to report for a parole meeting and was sent back to jail. He returned to prison for another year and released in 2009.

"I wasn't able to adjust after being alone and gone for so long," he says.

Bunn's prison identification card.

But Bunn was about to get some long-awaited good news. In 2010, the Exoneration Initiative, a nonprofit organization that provides free legal assistance to the wrongfully convicted in New York, began looking into Bunn's and Hargrave's convictions (Hargrave had not been granted parole and remained behind bars).

And on top of that, many of Detective Sarcella's convictions from the 80's and 90's were beginning to crumble, one by one.

In April 2014 came a breakthrough. A new motion was filed by Rosean Hargrave's legal team to vacate the charges.

"The pattern and practice of Scarcella's conduct which manifest a disregard for rules, law and the truth undermines our judicial system and gives cause for a new review of the evidence," declared the judge in the case, Justice ShawnDya Simpson.

Judge Simpson overturned Rosean Hargrave's convictions and ordered a new trial. After 24 years in prison, Hargrave was released on bond, pending retrial.

A year later, after a similar motion was filed on John Bunn's behalf by the Exoneration Initiative, his own day in court came. In her ruling, Judge Simpson called the evidence used to convict Bunn "paltry" and slammed what she called Scarcella's "disregard for rules, law and truth."

In May of 2018, after 27 years of wrongful conviction, the District Attorney's office announced they would not retry Rosean Hargrave or John Bunn. They became the 12th and 13th men to be exonerated of convictions related to investigations by Detective Louis Scarcella.

After Judge Simpson announced he was a free man, Bunn addressed the courtroom.

"I am an innocent man, Your Honor, and I have always been an innocent man," he said, tears streaming down his face, as his mother, Maureen, audibly gulped back sobs.

John Bunn holds hands with Judge ShawnDya Simpson at the hearing where he was exonerated after 27 years.

To date, 70 of Detective Scarcella's cases from the 80's and 90's have been flagged for review. And at least 14 of Scarcella's homicide convictions have been overturned because of tainted evidence, misleading testimony or forced confessions, according to the Brooklyn District Attorney's office.

But it's unlikely that Scarcella will face any consequences. He has never been charged with a crime. According to the DA, prosecutors have not found any laws that can be prosecuted today. The statute of limitations has expired for any potential criminal offenses.

CNN's attempts to reach Detective Scarcella were not successful, but he told the Daily News in 2018 that he had done "absolutely nothing wrong."

Scarcella remains in New York City, living on Staten Island. He still collects a full NYPD pension.

A new chapter

There's a palpable sense of excitement in the classroom at Ember Charter School in Brooklyn, where 25 third-graders await that morning's class speaker. When John Bunn walks into the room, after a rousing chorus of "GOOD MORNING JOHN BUNN," the class tumbles around him, lining up for hugs and high fives.

It's his second visit. He says the kids were so great the first time and had such smart questions that he had promised them he would come back. This time he's bearing presents -- tiny T-shirts in an array of colors, each with a logo that spells out the name of his new non-profit organization, "A Voice 4 The Unheard."

Bunn's project began as a book drive aimed at refurbishing the libraries at Rikers Island and providing under-resourced communities with educational literature. So far, he's donated over 20,000 books to help others affected by illiteracy and incarceration.

John Bunn leads a class at Ember Charter School in Brooklyn.

"Reading changed my life," Bunn says. "I want to share that experience with other people."

Along with starting a library on Rikers, he also works at the prison twice a week, running group sessions with 16- and 17-year-olds. He's also starting a book club for the teens and leading group discussions. More than anything, Bunn doesn't want the young boys he mentors to give up and become institutionalized.

"I explain to them, 'Anything is possible. Feel like you're too good to be in here!'" he says.

Back at Ember Charter School, Bunn is leading a class discussion.

Bunn receives hugs from third-graders after leading a class at Ember Charter School.

"Who here can tell me what peer pressure is?" Bunn asks the class, as a staccato wave of small hands shoots into the air.

He guides the children through a question-and-answer session, then doles out a bunch of shirts in pink, yellow and black. The kids swarm Bunn when the session is over.

"Let's get a group hug, y'all," he says as the kids clamber on chairs to get closer to him. Bunn's eyes fill with tears.

"There's no greater feeling than me feeling like I'm existing for a purpose, and this is what gives my life purpose right now," he says.

"Through my nightmare ... I found my dream."