Massachusetts native Mark Sullivan has a new World War II novel, based on a true story

Mark Sullivan, author of "The Last Green Valley."

Mark Sullivan might have sold you (or your parents) a Red Sox hat in the ’70s. As a Medfield teen, he worked a souvenir cart near Fenway Park.

He’s 62 today and living in Bozeman, Montana. He’s also a bestselling author.

While the Medfield High grad has written a few thrillers — including the “Private” series with James Patterson — his knack for World War II stories launched him to the top of the bestseller lists.

His 2017 novel “Beneath a Scarlet Sky,” based on the life of Pino Lella, now 92, tells the story of a teenage Allied spy working as a driver for one of Hitler’s generals.

A tip from a retired dentist at a book talk in Bozeman led Sullivan to the Martel family and the material for his next World War II novel.

“I knew within an hour that I was going to write it,” Sullivan says of their first meeting.

The result, “The Last Green Valley,” hit shelves this month and is a No. 1 Amazon bestseller in biographical literary fiction.

Ethnic Germans living in Ukraine in 1944, Emil and Adeline Martel face a decision as Stalin’s forces move back into Ukraine. Do they wait for Soviet intrusion and risk being sent to Siberia, as both of their fathers were? Or do they follow the Nazi SS officers conveying ethnic Germans out? The couple and their two little boys, Bill and Walter, are among the last Ukrainian Germans to leave in an exodus known as “The Long Trek.”

I caught up with Sullivan to talk about his latest bestseller.

Q. Can you explain why the Martels faced the decision to leave with the Nazis?

A. They were ethnic Germans [whose ancestors] had lived in Ukraine for more than a century, and life was good for them until 1917 and the Russian Revolution. Over the next 20 years, the family was thrown off lands ceded to them by Russia in the late 1700s— they were starved, beaten, oppressed and several of them were sent to Siberia and the Gulags. One never returned. One returned a broken man.

Under German occupation the Martels were given back their lands, treated relatively well, and prospered for almost two and a half years. With Stalin’s imminent return, the Nazis, they felt, were the best choice of two horrible choices.

Q. With Bill and Walter Martel — who were 6 and 4 when their family fled — you embarked on a month-long trip, retracing the Martels’ route from Ukraine to Poland. What was that like?

A. It really brought their ordeal to life for me. We drove seven hours up a horrible road to get to the little village where the brothers had lived before running with their parents in the last year of the war. We found the ruins of their farmhouse. Seeing Bill and Walter confronting how far they’d been blessed to come was stunning and emotional.

We chartered a plane to Poltava, where Emil was held in a Soviet Prison Camp. We found the building where Emil was held. The sheer enormity of what Emil had faced, endured, and overcome had us all in awe and on the verge of tears.

Q. How did you track down survivors, and people to interview?

A. I hired great guides in each of the countries I traveled through. I usually look for a guide with a journalism background. In Romania, for instance, my guide found nine people in their 90s who were either part of the Long Trek or remembered seeing it.

Q. Tell us about growing up in Medfield and your path to writing.

A. The high school was fantastic. My English teacher, Estelle Stahl, was the first person to tell me I could become a professional writer. I loved going into Boston to work the games at the souvenir cart. I come from a family of rabid Red Sox fans. I cried when they finally won the Series. But I can get Maine lobsters and Sam Adams here in Bozeman, so Montana’s not so bad.

Sullivan discusses his new book May 19 at 4:30 p.m. on Amazon Live. Interview has been edited and condensed.

Lauren Daley can be reached at [e-mail]. She tweets @laurendaley1.