Harmon's interest was initially sparked by a television programme on the Scarpa dynasty. On a whim, she wrote a letter to Scarpa Jr and was astonished to receive a reply. "It was a nice, sweet letter," she recalls. "He writes with smiley faces on the page. He's not at all tough. He would talk about his children and how much he loved his grandchildren, how much he missed Italian food like lasagne and his mother's cookies."
Harmon went on to become one of Scarpa's most trusted confidantes and yet the two of them have never met. Scarpa lives in a 7ft by 9ft foot cell in a maximum-security prison in Florence, Colorado and spends 23 hours a day in solitary confinement. He is not allowed visitors and is due for release in 2033, by which time he will be 84 years old.
Mafia Son tells the extraordinary story of Scarpa's betrayal at the hands of his father, Gregory Scarpa Sr, a brutal mafioso godfather nicknamed "the Grim Reaper" who led a double life as an FBI informant. Scarpa Sr was an enforcer for the notorious Colombo family, one of five Mafia families that controlled organised crime in New York through the 1970s and 80s. Stockily built and 6ft tall, Scarpa Sr ruled by fear: he took such delight in killing that he once expressed the desire to dig up one of his murder victims "and shoot him again". Yet he was also passing information to federal agents.
"He was a horrible, Machiavellian man who murdered over 50 people," says Harmon. "If anyone got too close, he murdered them. He was vicious."
His eldest son, Greg Jr, was raised in a similar mould. "He didn't know any different," says Harmon. "It was the family business and Greg Jr idolised his father. He loved him so blindly. He told me that he didn't like the violence at first, that it made him feel sick but it was a job he had to do because if he'd wanted out, his father would have killed him.
"When I spoke to his mother, she said Greg Jr used to help her in the kitchen and was a sweet little boy who wanted to make everybody happy."
In 1987, Gregory Sr was tipped off that the authorities were closing in on his expanding narcotics operation. Worried that he would die in prison after having contracted HIV from a blood transfusion the previous year, he persuaded his son to take the rap for him.
"He told him 'You're young, you'll only get a couple of years,'" says Harmon. "Greg Jr told me he didn't want to do it because he had a wife and a baby and was happy with his life, but he couldn't say no to his father. He took the fall and the sentence was 20 years [he later had his sentence increased on racketeering charges]. His father had made it look like his son was the boss and Greg Jr never got over the betrayal. He goes over it in his mind all the time. He remembers how his father used to coach him in Little League and he still can't understand it."
Over the five years of their correspondence Harmon got very close to Scarpa Jr. "Ultimately, I sent him my picture and I think he fell a little in love with me," she admits. "He said he had my photograph on top of his TV."
Does she feel her own objectivity was threatened? "Yes. It was challenged. For a long time, I believed him when he said he'd never killed anyone [it later transpired that Scarpa Jr was responsible for 26 murders]. But he's still a human being. I can't say what he did was evil. He needed to prove he was his father's son."
In jail, Greg Jr's story took an even stranger twist when he found himself in a cell alongside the al-Qaida terrorist Ramzi Yousef and befriended him, believing that any valuable information he gleaned could be passed on to the authorities in the hope of reducing his sentence. Scarpa claims that Yousef told him in 1996 - five years before 9/11 - about an al-Qaida plan "to bring New York to its knees" by blowing up the World Trade Centre. Although there are official FBI memos that confirm Scarpa's intelligence, says Harmon the information was buried. "They chose what was easiest. They said 'He's a scumbag, he's no good. Why would we believe him?'"
Harmon admits that her relationship with Scarpa started to dominate her life. During the writing of Mafia Son, she was subpoenaed in the highly publicised 2007 trial of Lindley DeVecchio, a former FBI agent accused of plotting four gangland murders with Scarpa Sr. "I've got no regrets [about the book]," she says. "But would I do it again? Probably not. These have been very hard years."
Greg Scarpa Jr has not yet seen a copy of the book he helped to write. "But I sent him the cover and some of the reviews." What did he think? Harmon laughs. "Oh, he thought it was dynamite."
