One summer day 10 years ago, Rachel Cockerell gathered with dozens of family members for a cousin’s 80th birthday party in the North London house where her father had grown up.

Among the guests were relatives from Israel. Cockerell had always known that alongside her impeccably Anglo ancestors, she had descended from Russian Jews through her father’s mother. But she never knew her Granny Fanny; she had never celebrated Jewish holidays. A perennial jar of borscht in the cupboard had been the extent of her Jewishness. “It was so peripheral in my vision,” she said recently.

Something about spending the afternoon with Hebrew-accented cousins in the overgrown backyard of 22 Mapesbury Road sparked her imagination. Cockerell, who is now 30, Googled this branch’s paterfamilias, her great-grandfather David Jochelmann. Up came his New York Times obituary from 1941. It stated: “His name was a household word in Jewish homes throughout Eastern Europe.”

That one sentence turned out to determine how she would spend much of the next several years of her life.

Prowling century-old newspaper articles and digitized memoirs, Cockerell put Jochelmann’s story at the heart of what became her first book, “Melting Point: Family, Memory, and the Search for a Promised Land,” published last year in Britain and May 6 in the United States.

“This man, my great-grandfather, was never spoken about in my family, and if he was, he sounded deathly boring — a ‘gray man,’ as we say in London,” Cockerell, who grew up in Notting Hill, said over lunch in Brooklyn last week.

But Jochelmann (spelled Jochelman in some sources) turned out to be the Zelig of early Zionism, which began as a political movement to address Jewish persecution roughly a half-century before Israel’s founding in 1948. He worked closely with Israel Zangwill, the writer and activist who, in a 1908 play, coined “the melting pot,” the trademark phrase of American assimilation.

Jochelmann helped execute Zangwill’s grandiose scheme that, beginning in 1907, relocated thousands of Russian Jews to Galveston, Texas, on the Gulf Coast. Decades later, in England, Jochelmann was personal assistant to Vladimir Jabotinsky, the intellectual godfather of the Zionist right. His three children lived fascinating lives of their own in the United States, Britain and Israel.

“Melting Point”(Farrar, Straus & Giroux) is not sui generis so much as multigenre: partly an immersive history of major events (early Zionism and the schism within its ranks); partly a nonfiction novel of ideas; partly a caper among fast-living bohemians; partly a family saga; and ultimately Cockerell’s reclamation of her birthright.

“When I have been at Passover or in synagogues or at Hanukkah in Israel and America, I’ve seen what my family could have had, but didn’t,” she said. “This rich, ancient heritage of ritual, this culture that had been passed down however many years, ended with my grandmother or my dad — it definitely was not something that I inherited.”

In fact, much of Jochelmann’s story was new even to Cockerell’s father, Michael, a prominent British political journalist who grew up with a portrait of his grandfather staring from the wall of 22 Mapesbury.

“It feels very odd to have a book written about my family without me really recognizing much of what I knew,” Michael said in a phone interview.

He was particularly struck by the book’s documentation of the antisemitic attacks that helped spur both Zionism and efforts such as what came to be called the Galveston Movement. “There is a lot of my own ancestry,” he said, “I hadn’t realized was as awful as that.”

Rachel Cockerell is less a presence than she might have been because of her book’s unusual style. After composing half a draft in the manner of a typical nonfiction volume, she cut all her own prose.

“Melting Point” has a short preface, a brief afterword and some reproductions of faded photographs and paintings. It otherwise consists entirely of quotations: from newspaper articles, from speeches, from writings of obscure figures, from interviews Cockerell conducted with family members. Separated into blocks, they last anywhere from one sentence to a few paragraphs and are adorned with minimalist sourcing in the margins: “Chicago Tribune, 24 August 1903,” “Weymouth Gazette,” “Israel Zangwill,” “Mimi.”

Cockerell found that attempts at paraphrasing just didn’t work for her. Inspired by the George Saunders novel “Lincoln in the Bardo,” which is organized as a kind of oral history, she strove to make the book personal through authorial curation.

“My favorite thing people have said to me after reading the book,” she said, “is, ‘It feels like it’s written in your voice.’”

The Guardian called the method “deeply immersive and dramatic.” The New Yorker described how effectively sources were “coaxed by Cockerell, who has a keen ear and fine sense of timing, into becoming some of recent literature’s most compelling narrators.”

Substantively as well, “Melting Point” is not the book Cockerell set out to write after that birthday party in 2015. She was expecting to compose a Jewish family memoir along the lines of Edmund de Waal’s “The Hare With Amber Eyes.”

But Cockerell, who studied art history as an undergraduate at the Courtauld Institute of Art and received a master’s degree in journalism at City University, found her attention increasingly captured by the larger currents of Jewish history in which her great-grandfather played a first marginal, and then increasingly prominent, role. It would be necessary, she felt, to explore beyond the exclusive lens of her own family.

And so the book’s first half revolves around Zangwill and his notion of “territorialism,” which diverged from Zionism’s insistence on a Jewish territory in the Jews’ ancient homeland. Prompted partly by an infamous 1903 pogrom in the Russian city of Kishinev, territorialists held as an intermediate goal a Jewish territory anywhere one could be gotten — “Zionism without Zion,” as The Jewish Chronicle described it.

Locales that were considered included East Africa (“Is it to be Jewganda?”), Australia, Mexico, Mesopotamia, Paraguay, Canada and Angola. A then-Ottoman part of modern-day Libya called Cyrenaica was ultimately rejected over a lack of water (“An Unpromising Land,” declared The Evening Standard).

Galveston, a Gulf Coast port, was conceived not as a Jewish territory but as an entry point to the interior United States, as opposed to the East Coast cities — above all, New York — where Russian Jewish immigrants were already concentrated.

Territorialism is a decidedly less-known aspect of Zionist history. In an interview, Adam L. Rovner, the director of the Center for Judaic Studies at the University of Denver, called Zangwill an “amazing, imaginative man,” whose vision fell out of favor. “His pragmatism,” Rovner added, “blinded him to the pursuit of the dream that has had undeniable success.”

Though not Cockerell’s relation, it is Zangwill who above all leaps off the pages of her book. He was “the homeliest man I ever saw,” according to one source; “negligent of his apparel,” according to another; and “with a face that suggests nothing so much as one of those sculptured gargoyles in a medieval cathedral,” in the words of The New York Herald. (The descriptions could veer into antisemitism.) Yet he was tremendously charismatic.

And also pragmatic, not only as a territorialist but as a mentor and a friend. In 1913, he urged Jochelmann to leave Kyiv, not for one of Zangwill’s hoped-for Jewish territories, but for London. “They are destined to become English,” Zangwill insisted of Fanny, Cockerell’s grandmother, and her sister, Sonia.

Following Zangwill’s story, the book shifts to 1920s Greenwich Village, where David’s son Emanuel Jochelmann ended up a playwright under the name Em Jo Basshe. Though he worked in an experimental collective with John Dos Passos, he achieved less renown: One review remarked of the unfortunate hero of a Basshe play, “He suffered almost as much as his audience.”

To capture this branch of the family, Cockerell spoke with Basshe’s daughter, who is now 95 and lives in Canada, nearly every week for two years.

The book goes on to explore the middle-class lives of Jochelmann’s two daughters and their children in postwar London and Israel. Capital-H history is glimpsed in passing: a London protest against Hitler in 1933; the 1946 bombing of Jerusalem’s King David Hotel by a Jewish militia; an older woman’s reminiscences of herself as a young girl spying telltale arm tattoos on Israeli bus passengers.

A sub-current throughout “Melting Point” speaks to present-day debates over what might be called the Palestinian question. “The principal difficulty is that Palestine is already the homeland of another people,” one source observes; at the close of the 1940s another reports: “The once all-Arab cities of Jaffa, Ramleh and Acre are now filling up with new Jewish immigrants.”

The stories and quotations bump and jostle, leaving the reader to decide what might be the book’s central tension: How does its first half — the history of Zionism and its failed alternative, territorialism — connect to the second half, which depicts a Jewish family’s assimilation?

As part of her reporting, Cockerell celebrated Passover and Hanukkah for the first time, in Israel. She ate fish tacos on the Galveston beach with a longtime local rabbi and his wife, a descendant of Jews who arrived there a century earlier as part of her great-grandfather’s movement.

“My family has melted into the melting pot, and I can’t be too sad about that,” Cockerell said. “Because I am the textbook example of assimilation, I can’t resent it.”

For now, she is done looking back at her roots; her next book is about Halley’s comet. “I feel bad for the gentile side of my family,” she said. “My uncles on my mom’s side asking, ‘When are you going to write about us?’ It’s like: Absolutely never. Sorry, guys.”