In Unmasked: My Life Solving America’s Cold Cases (Celadon, Apr.), Holes describes the emotional toll of pursuing murderers for decades.

When did you start thinking about writing this book?

Right before I retired—the book was to be a deep dive on the Golden State Killer investigation, which was close to being cracked. As my agent and publisher started to learn more about me, the book evolved to not only talk about the cases but about me opening up about myself, which I’d never done. But I’m very pleased, because I think people can learn about what happens to individuals who work in this type of career, and the sacrifices we make as we work through it.

Was the hardest part putting your personal struggles out there?

No question. I’m naturally a very private person. And when I’m being asked, can you talk more about your relationships, and what you are feeling when you are dealing with these types of cases, it felt like I was exposing myself.

You wrote that at times you’ve “visualized dead women during intimate moments.” What help did you get to be able to deal with all this violence and pain?

Seeing women’s bodies having been mutilated by a serial predator had an impact on my personal relationships. Carla Walker, for example, was a 17-year-old girl, and I’m having to sit and stare at what the offender did to her, and try to reconstruct in my mind what she was going through, and then update the family. When I started sobbing after I talked to her brother, I realized that there’s something wrong with me. And that’s when I went to a therapist who specialized in treating individuals traumatized through violence. And when I talked to her about what I was experiencing, she said, this is a form of trauma that’s like tiny little cuts that, over time, pretty soon add up and you start bleeding out, and that’s what I was experiencing. And so I went through therapy.

What was the most valuable contribution made by the late Michelle McNamara, author of I’ll Be Gone in the Dark, to the investigation of the Golden State Killer case?

The most significant was her ability to obtain all the files from the Orange County Sheriff’s Office, because Orange County had stuff I never saw. Michelle had access to it, and passed on information about homicides that that agency had never shared. And that was so critical. Michelle was able to break through walls that I wasn’t able to break through, despite my being a member of the GSK task force.