The subtitle of Dangerous Ground, the latest from award-winning investigative journalist and New York Times bestselling true crime writer M. William Phelps, is My Friendship with a Serial Killer. But is it really possible to be friends with a serial killer?

Phelps says he’s using the term friends loosely. “I viewed our professional relationship as reporter vs. psychopath, but there were times when I felt there was a breath of a human being trapped deep inside this killer’s mind.”

Dangerous Ground tells the story of that interplay between the human and the psychopath. It’s an unusually personal book for Phelps, who has written more than 20 true crime books to date. Like his other work, Dangerous Ground offers readers a fascinating glimpse inside the mind of a criminal—but it also puts Phelps squarely in the story, exploring how his relationship with the killer develops. It’s as much a memoir as a true crime book.

Phelps first contacted the killer, whom he calls Raven, in 2011 while working on his television series Dark Minds for the Investigation Discovery network. Raven served as a consultant on the show, which examined unsolved serial murders. He offered insight into the crimes, but his identity was concealed from viewers. (It is revealed in the book.)

Over the course of five years, Raven sent Phelps thousands of pages of letters and they spoke for hundreds of hours by phone and Skype. Phelps also met Raven at the state penitentiary where he’s serving two life sentences for murdering several women. Their encounter is described in vivid detail in the book.

Phelps says of meeting serial killers: “They are able to come across as any other person you might meet on the street, at the post office, or in the supermarket. But also, when talking about the most despicable, graphic aspects of murder, the tone they use sounds as though they’re talking about a recipe they might want to cook. There’s a resigned, glassy polish about them that you can only experience in person—and part of it, believe it or not, draws you in.”

At first Phelps despised Raven, but then something happened. “I began to experience the faint rustlings of a human being behind Raven’s dark persona,” he says. “Which scared me, but also inspired me to dig deeper into his mind.”

Phelps approached his subject not just as a journalist but as someone whose own life has been touched by a murder. In 1996, his sister-in-law, then five-months pregnant, was strangled—a crime that remains unsolved. “As someone who has lost a family member to murder, I think I can step into the shoes of a murder victim’s family members, sympathize, and grasp their pain,” he says.

As Phelps got to know Raven, the killer offered to share his own dark insight on Phelps’s sister-in-law’s case. Raven also told Phelps about one of his victims who hadn’t yet been identified by police. Over the course of their many conversations, an unlikely bond developed between the two—a bond explored in vivid detail in Dangerous Ground.

“In order for readers to fully grasp the manipulation, the charm, and the mixture of personality disorders behind the serial killer’s mask of sanity, the story had to be told from my point of view,” Phelps says. “I can assert with absolute confidence that by the end, readers will understand this serial killer and how his mind works.”

The result is an engrossing read, but it came at a cost to the author. “Writing this book broke me: physically, spiritually, psychologically,” Phelps says. “It took a part of my soul I will never get back. I also left a part of myself on the pages I do not want back. A funny thing happened while writing this book: I learned a lot about myself, my past, my family. It was all in me, yet forcing it out on the page allowed me to see it clearly for the first time. And what I saw, I did not always like.”