One day, my father fell out of an envelope into my hands. It was a regular brown mailing envelope, bulging in the center, with writing on the front in my aunt’s hand: Dicky — Vietnam.

I was standing at my desk in front of a lamp, and when I tipped the envelope to slide out its contents, one of the first things that came out was a color slide photograph. I held it to the light. A young shirtless man, on a beach with the dark jungle in the background, not quite smiling, not quite frowning, looking just tough enough. My father, age nineteen, Marine Air Guard Private First Class, in Chu Lai.

The envelope held a few more slide photos of him and his buddies, a pair of round, John Lennon-style glasses, and a packet of letters that my dad had written home to his older sister, my aunt. I’d never seen these letters before, never even knew they existed until my aunt loaned them to me and my sister.

As much as I wanted to, I couldn’t hold on to the packet forever—they belonged to my aunt, and we all had so little left of my father that I could hardly blame her for wanting them back. So I did what any writer would do: I sat down with my father’s letters and retyped them all for myself.

When my father died at age 47, I was 21 years old. I loved him, of course—he was funny, tender-hearted, generous—but I’d also come to fear and resent the parts of him that I couldn’t understand yet from my perspective as his kid—the deep anger, the PTSD and depression, the tendency to violence.

As I read and retyped these letters from a 19-year-old Marine on active duty in Vietnam, I was re-recording and playing back their heartbreaking mix of terror (here’s a teenager, new to combat and enduring repeated mortar attacks), determined bravery (true to Marine Corps code; semper fi, do or die) and helpless concern about what was going on at home with his sisters, halfway around the world. His family was close, but in some ways troubled, as so many families are, and his younger sister disappeared not long after he came home from the war.

At the time that the envelope landed in my lap, I’d been working on a novel set in a near-future world, about a young veteran of wars spurred by environmental collapse. I was still in the process of figuring out who the main character was, what motivated him, what his real fears were.

As I retyped my father’s letters home from war, I felt like I was not just discovering my characters’ fears and motivations, but what my dad’s had been as well. What could it have been like to go to war as a teenager, what could it have been like to live with that kind of fear—while also living with so much fear about your family’s safety back home?

The main character of the book I ended up writing, The Completionist (Simon & Schuster), is a young veteran who returns from war to solve the mystery of his sister’s disappearance, uncovering painful secrets in the process, is based in part on my father’s experience. The novel incorporates excerpts from his letters home while building a different context around them.

In its own personal and flawed way, the act of writing a story based on my dad’s experience turned into an attempt to bridge the wide divide between his life and mine—combat Marine and civilian, adult and child, father and daughter. It’s a gap that I continue to try to reach across, many years after his death, in my work as an instructor for a veterans’ creative writing workshop, and in the ongoing work to raise a daughter who understands the pride, pain, and honor that my father’s memory represents for me.