Sylvia Jukes Morris, a British-born biographer of first lady Edith Roosevelt and playwright-diplomat Clare Boothe Luce, whose lives she chronicled in lucid prose and meticulous detail, died Jan. 5 in Bridgnorth, England. She was 84.

The cause was cancer, said her sister and sole immediate survivor, Pauline Pennington. Mrs. Morris died less than eight months after her husband, fellow biographer Edmund Morris, who was 78. A resident of Kent, Conn., she was staying with her sister in Bridgnorth for Christmas.

Mrs. Morris came to biography somewhat late in life, while helping Edmund prepare his first book, “The Rise of Theodore Roosevelt” (1979), which won a Pulitzer and National Book Award. The Morrises had previously collaborated on travel cassettes for Trans World Airlines, researching European history and culture for jet-setting tourists. It was unsatisfying but life-altering work, Mrs. Morris told The Washington Post last year. “We realized that we were biographers at heart.”

Neither had graduated from college, although both had literary temperaments; Edmund said that he fell in love with Mrs. Morris after spotting a worn leatherbound set of Boswell’s “Life of Samuel Johnson” in her apartment. She was the kind of reader who devoured five books a week and could rip through a 500-page volume of Harold Nicolson’s diaries on a 3 ½ -hour car ride — then recommend a certain passage from memory, her sister said, “on page 37, three-quarters of the way down.”

Mrs. Morris helped research and edit “The Rise of Theodore Roosevelt,” which spurred her interest in the former Edith Kermit Carow, who married Roosevelt in 1886 after the death of his first wife. “There’s not much known about her,” Edmund said, which Mrs. Morris seemed to take as a challenge, poring over the first lady’s letters, diaries and papers to write “Edith Kermit Roosevelt: Portrait of a First Lady” (1980).

“If Theodore Roosevelt’s turbulent life can be described as cyclonic,” she wrote, “his wife was the still center of the storm. Reserved to the point of aloofness, and given to the sternest self-discipline, Edith was a lifelong enigma even to members of her family. Yet no one denied her power and influence. She was ‘a sort of feminine luminiferous ether, pervading everything and everybody.’ ”

Mrs. Morris’s subject had died 32 years earlier, but her 600-page biography benefited enormously from interviews, notably with Edith’s 85-year-old son, Archibald Bulloch Roosevelt, who responded to a request for information Mrs. Morris had placed in the New York Times Book Review.

A close relationship Mrs. Morris forged with his son Archie Jr. launched her on a biography of Luce, who rose from a tumultuous childhood in Spanish Harlem to become a Vanity Fair editor, congresswoman, diplomat and playwright, as well as the wife of Time magazine co-founder Henry Luce.

Mrs. Morris met Luce at a Roosevelt family dinner party. “She appeared to pay no attention to me,” Mrs. Morris wrote in a New York Times essay, but the 77-year-old Luce later remarked: “You’re too young and charming to be a writer.” Nonetheless, the two began a courtship through the mail, with Mrs. Morris sending a copy of her last book and writing, “Someone will embark on a major biography, and I should like it to be me.”

The biography ultimately spanned 1,300 pages and two volumes, “Rage for Fame: The Ascent of Clare Boothe Luce” (1997) and “Price of Fame: The Honorable Clare Boothe Luce” (2014). Beginning her research in 1981, Mrs. Morris devoted 33 years to the project, working her way through 246 linear feet and 460,000 items of archival material at the Library of Congress — a collection, she said, that was “bigger than that of most Presidents.”

“Like her husband, she was an indefatigable researcher, and wanted to leave no stone unturned,” said Will Murphy, who edited her second Luce volume for Random House. “Sylvia was trying to absorb into the life of Clare Boothe Luce her times, and the history that unfolded around her and that she helped shape.”

Luce had — at 20 — married a millionaire who was more than twice her age, then divorced him and landed a job at Vanity Fair after simply showing up at the office, sitting down at an empty desk and waiting for assignments. She went on to write the hit 1936 Broadway play “The Women,” which became a popular film, and struck Henry Luce like a bolt of lightning.

According to Mrs. Morris, he left his wife and two sons after seeing Clare for the second time; later, Luce served two terms in the U.S. House of Representatives and was appointed Eisenhower’s ambassador to Italy and Brazil. “I hope I shall have ambition until the day I die,” she told Mrs. Morris, in one of their many interviews before Luce’s death in 1987.

Both volumes of Mrs. Morris’s biography were met with general acclaim, with Gore Vidal calling “Rage for Fame” — the title came from a quote by English satirist John Wolcot, which Luce used for her eighth-grade yearbook — “a model biography . . . of the sort that only real writers can write.”

The books also angered some reviewers, and friends of Luce’s, who accused Mrs. Morris of being too critical in her examination of a woman described as an inveterate embellisher of stories, and sometimes as an outright liar. (She also had a playful, adventurous side, Mrs. Morris reported, which included scuba diving and LSD use.)

“These characters take over your life, these huge characters,” Mrs. Morris told Newsday in 2001. “I find that you neglect your personal life, your family life.” But it took time, she said, to uncover details such as the color of Edith Roosevelt’s eyes (sapphire blue) or the humorous names Clare Luce gave to her ulcers (Qaddafi and Begin).

“The longer you take,” she added, “the better understanding you have.”

Sylvia Jukes was born in Dudley, a suburb of Birmingham, on May 24, 1935. Her mother was a homemaker, and her father ran a munitions factory. Mrs. Morris “was very misunderstood in the area we were brought up in,” said her sister, Pennington. “She was a very quiet little girl, basically, and I think people found that a little intimidating.”

Mrs. Morris was 9 when her mother died. She won a scholarship to a girl’s high school and drew early praise as a writer for her senior thesis, which was read by historian A.J.P. Taylor, according to Pennington, and traced the 19th-century deaths of local children to issues with the town’s sewage system. She later studied at the University of London but left before receiving a degree, her sister said.

Mrs. Morris was teaching English at a London high school, living at a nearby boardinghouse, when she met Edmund. Raised in South Africa, he was an advertising copywriter and devoted pianist, and developed an informal arrangement with Mrs. Morris’s landlord, performing chores in exchange for practice time on the building’s piano.

They married in 1966, after she spotted him washing the floor, and soon immigrated to the United States. His Roosevelt project grew into a trilogy — almost twice the length of her Luce books — and both worked closely together. She read him each page of her manuscripts, including a passage on Theodore Roosevelt’s death that had moved her to tears and left stains on her handwritten draft.

“They felt things so deeply,” said Marion Elizabeth Rodgers, an H.L. Mencken biographer who worked as a researcher for both Morrises in the late 1980s. “To write these books, it was gut-wrenching. It took a lot out of them, physically and emotionally. . . . Edmund always said you have to be something of a masochist to be a biographer, and I think that’s how she felt, too. But once she started writing, she could get on a tear: She’d begin in the morning, and wouldn’t even get up to have lunch.”

In a C-SPAN interview, Edmund Morris recalled that he was once asked by a Times reporter whether it was easier to write about the living or the dead. “I hadn’t thought of it, and I was struggling to think what to say, when my wife, who was in the next room, shouted out, ‘Dead is easier.’ And of course this journalist used that line to begin his article in the New York Times the following day.”

Mrs. Morris, he added, “got a T-shirt a couple of days later, which has ‘Dead Is Easier’ all over the front and which she wears when she gives lectures.”