When journalist and author Andrew Meier set out to write a history of the Morgenthau family, he knew it would be no small undertaking. He estimated that writing the story of the American political dynasty would take five or six years.

Instead, it took 15 years.

The journey began in 2009, when Mr. Meier conducted his first interview, and was completed in 2022 when his book Morgenthau: Power, Privilege, and the Rise of an American Dynasty was finally published. It is a massive undertaking in generational history, a work stretching to more than 70 chapters and, with the bibliography, more than 1,000 pages.

For a family of such stature, the length is fitting. Spanning four generations and three presidential administrations, in their roles advocating against the Armenian genocide and the Holocaust, from a failed immigrant businessman to the longest-serving district attorney in Manhattan history (and seasonal Vineyard resident), Mr. Meier’s book tells the story of the nation through one family.

“Writing is a solo practice, it’s transatlantic sailing,” he said of the long process. “A lot of times you’re just navigating from the stars and you really can’t see land at all . . . . I’d be lying if I were to tell you that it wasn’t incredibly frightening.”

Though Morgenthau is an eminently American story, Mr. Meier’s career began in Russia. He spent a decade there, first as a foreign correspondent for Time Magazine, then researching his first book, Black Earth.

“I traveled the whole former Soviet Union,” he said of that project. “It was about the first post-Soviet decade and the rise of a guy called Vladimir Putin.”

Returning to America, Mr. Meier wrote his second book, The Lost Spy, while also writing about New York City politics for The New York Times Magazine. It was in that capacity that Mr. Meier encountered district attorney Robert Morgenthau, then running for his ninth term in office.

“He was 90 years old and he had the slogan ‘90 in ’09,” Mr. Meier said. “People called him the D.A. for life.”

Mr. Morgenthau initially wasn’t interested in being interviewed for a magazine article, Mr. Meier said. However, when Mr. Meier mentioned that he had read the German-language diary of the D.A.’s great-grandfather, Mr. Morgenthau became more interested.

“When I said that, his ears perked up. He was a little more interested in what I call the long arc of history,” Mr. Meier said.

And so, with the D.A.’s blessing, Mr. Meier set off on a journey that led him to 1866, when great-grandfather Lazarus Morgenthau first landed in America.

But mapping out a course for that journey, Mr. Meier admitted, was a daunting task.

“I immediately realized that there was going to be not a mountain of documents, but several mountains,” he said. “There are literally millions and millions of pages of state archives, private archives, diaries, letters, official correspondence . . . and it’s no exaggeration to say that the Morgenthaus are in thousands of books.”

Over the course of writing, Mr. Meier conducted about 350 interviews.

In writing a work of such scale, Mr. Meier said he paid particular attention to how the book was organized, splitting it into five sections and numerous short chapters to keep readers engaged.

The first three sections focus on the first three individuals: Lazarus Morgenthau, a briefly successful then perennially-failing businessman, Henry Sr., a Manhattan real estate tycoon and Woodrow Wilson’s ambassador to the Ottoman empire who documented the Armenian genocide, and Henry Jr., the treasury secretary in FDR’s New Deal administration noted for his push to intervene in the Holocaust.

The career of Robert Morgenthau, meanwhile, spans the last two sections of the book, from his World War II career, to his long tenure as U.S. attorney for the southern district of New York who was appointed by his close friend John F. Kennedy, to his 34-year tenure as Manhattan D.A.

“He changed prosecution across the country,” Mr. Meier said.

Robert Morgenthau was also the one who brought the family to the Vineyard.

“He loved to fish for blues and he loved to sail,” Mr. Meier said, a love going back to summers spent with the Kennedy family on the Cape. Later, he became known on-Island for his 32-foot powered wooden boat, Souvenir.

Putting the story of the four men together, Mr. Meier said, was a delicate balancing act.

“I knew it was still going to be massive and at some point it has to get between two covers,” he said, noting that the key is to know which details to keep and which to cut.

“Biography is really the art of subtraction,” he said. “You need to find the telling details, and it’s often the things untold that capture the reader’s imagination and keeps things moving along.”

Andrew Meier will take part in a panel discussion at 12:30 p.m. on August 5, and a conversation at 1:10 p.m. on August 6.