William Hassett was nicknamed “Bishop” by his boss, President Franklin Delano Roosevelt, both for his strong ethical sense and for his discretion.

That personality allowed Hassett to get to know the man in charge better than most. After working closely with FDR and then Harry Truman, Hassett published a book journaling his years in the White House. But now his great-nephew has blended those public papers with private ones and family recollections to write a historical novel, “The Yankee Sphinx,” chronicling Roosevelt’s and Hassett’s years together in the White House, especially during World War II.

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That great-nephew, Mark Frost, already has quite a track record as a writer, although his best-known works are as far from this world as you can imagine. As a writer for “Hill Street Blues,” he specialized in the earthy and verbose speeches of Andy Renko and Phil Esterhaus; later, he co-created and co-wrote, with David Lynch, every episode of “Twin Peaks” and “Twin Peaks: The Return.”

He has also written several murder mysteries, a return to his original medium. “I wrote a novel when I was 11, another when I was 13, and a third when I was 16,” he says. “Then I got involved with the theater, which gives you a sense of community and a chance to meet girls. So I started writing plays.”

Frost, whose family came to America in 1634, found out while writing the book that FDR is his seventh cousin. He did the interview wearing a Dodgers hat, although he noted that his nephew, Lucas Giolito, now pitches for San Diego.

“It’s paining me to root for the Padres every fifth day,” he says. He was also sitting beneath a framed image reading “Don’t Give Up the Ship,” noting, “I’m a Stoic and an optimist, and I don’t think those two things are contradictory — I don’t waste a lot of time fretting.”

This interview has been edited for length and clarity.

The real-life story behind ‘Twin Peaks’ co-creator Mark Frost’s FDR novel
President Franklin D. Roosevelt and Secretary William D. Hassett as F.D.R. leaves Poughkeepsie, New York on April 7, 1940, returning to Washington. (AP Photo/George R. Skadding)

Q: What drew you to write about your great-uncle?

I knew him when I was a kid. He had a wonderful, Gandalf-like presence — old and quiet and wise. I already knew I wanted to be a writer, and he was the only writer in the family, so I guess he took a shine to me. He gave me a copy of his book, which had been published in 1958, for my 11th birthday. I didn’t read it until I was in my twenties, but I’ve carried it with me my whole life. And because he was so close to my dad and particularly my grandmother, who lived to be a hundred, there was a lot of family lore about him that I liked. He was revered.

But this started because I was in New York on Election Day in 2016. The next morning, like many people, I didn’t know what to do. I’d met this guy; I knew what the score was. My response was to call the FDR library — my uncle was the original director of it — and they assembled this massive amount of material and gave me a tour of the grounds. And I walked out thinking I wanted to write about my uncle and FDR. Fortunately, I also had access to all of his private papers as well through other relatives.

I started by writing a play. I’d trained as a playwright but hadn’t written one in 40 years. I thought it was a great way to explore this relationship, and I spent four years writing and had staged readings with terrific actors, but then I realized I needed to tell the story in my uncle’s voice, with an interior point of view, the fly on the wall aspect that makes his role so compelling.

Then Biden won in 2020 and the urgency wasn’t there, but then January 6th happened, and I called my publisher the next day about turning this into a novel.

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Q: How did Will see himself in his role at the White House?

He was happy to give himself to the causes that FDR represented, and he understood him because he’d known Roosevelt since the teens. As a journalist, he’d been to a lot of places and seen a lot of things, so he had a sense of the moment and of history. He was uniquely well-equipped to serve as the only journalistic eye inside that circle. I also admired his rigor to his own standards. And he accepted that he was a keeper of secrets while keeping an eye on history, recording everything because they were momentous times, and he was seeing momentous things.

I had a chance meeting with Doris Kearns Goodwin in 2009 at a Red Sox game, and when I told her Will was my great-uncle, she grabbed my arm and said, “You have no idea how important he is to historians. He’s the only one who saw behind the mask of who Roosevelt was.” That carried a lot of weight with me when I was deciding to tell this story.

Q: You learned a lot about other major players like FDR advisor Harry Hopkins. What surprised you the most?

Anna Roosevelt, the president’s daughter, is a remarkable figure and one that history has not paid enough attention to. She was the one who had the guts to confront her father’s ridiculous doctor, the surgeon general, who knew nothing about what he was doing, but was too blustery to admit it.

Because of that, she extended her father’s life by a year-and-a-half during which some of the most consequential events of the war and of our history occurred. I’m not sure that would have happened without her. She may have saved the world.

Q: Both Anna and your great-uncle hid from Eleanor the fact that during his final years, the president resumed seeing Lucy Mercer Rutherford, Anna’s former nanny and FDR’s long-ago mistress and the love of his life. What did you make of that decision?

It’s  a classic case of, “What’s the greater good here?” It’s Stoic philosophy 101. He knew he was ill and/or dying, which is why he broke the deal he made with Eleanor not to see Lucy again. And for Anna and Will, the greater good was keeping Franklin going. In the final analysis, I would have made the same decision.

They kept it from Eleanor as long as they could, and if not for a dopey relative, she might never have known. It hurt her very deeply, but she and her daughter made amends, and in her own way, she made amends with Lucy, which is a testament to the sterling quality of her character and a hint at why she’s so revered.

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Q: FDR used your uncle’s pen to sign numerous bills. Do you know where it is?

It’s with my uncle’s papers at Northfield in Vermont. It’s very important to me because it was used to sign the GI Bill, which is how my dad got to go to college, which is how he met my mom. So that pen is why I’m here.