James Atlas, whose biographies of writers Delmore Schwartz and Saul Bellow were both acclaimed and attacked and who, as a publishing executive, ushered dozens of other biographies into print, died Sept. 4 at a hospital in New York City. He was 70.

The cause was a chronic lung ailment, said his daughter, Molly Atlas.

Mr. Atlas gained renown with his first book, “Delmore Schwartz: The Life of an American Poet,” published in 1977, when he was 28. Schwartz, a promising writer in the 1930s and 1940s, was perhaps best known for a short story, “In Dreams Begin Responsibilities,” written when he was 21.

In his youth, Schwartz was handsome, charismatic and the friend of countless literary figures, but he ended up as a down-and-out alcoholic, struggling with mental illness, dead at the age of 52. Drawn to the unfulfilled promise of Schwartz’s life, Mr. Atlas read every word he had written — even his incoherent scribblings later in life — and interviewed everyone still alive who had known him.

One of the people impressed by Mr. Atlas’s biography was Bellow, who had used Schwartz as the basis for the central character in his Pulitzer Prize-winning 1975 novel “Humboldt’s Gift.” After writing a poorly received novel of his own, “The Great Pretender” (1986), Mr. Atlas decided — at the suggestion of novelist Philip Roth — to return to biography and scale the literary mountaintop that was Bellow.

Winner of the Nobel Prize and often proclaimed the greatest novelist of the second half of the 20th century, Bellow was a formidable subject for any biographer — not the least because he was still alive.

Mr. Atlas spent 11 years on his biography, interviewing Bellow and his acquaintances at length and reading his correspondence, before his book was published in 2000. Even then, he had to make last-minute revisions when Bellow, then in his 80s, wrote a new novel and became a father for the fourth time.

“The biographer enters into a kind of relationship [with his subject] that’s duplicated nowhere else in life,” Mr. Atlas wrote in an introduction to “Bellow: A Biography.” “He becomes an instant confidant, privy to the most intimate secrets, a conduit for unresolved and unexpressed feelings; yet at the same time he’s supposed to remain emotionally distant, formal, objective.”

Critics were wildly divided over Mr. Atlas’s 686-page book.

“A biographer more scrupulous than Atlas is hard to imagine,” critic John Leonard wrote in the New York Times. “I could no more stop reading his biography than I could stop reading Saul Bellow after he blew the blinds off the windows in my head.”

Other reviewers thought it was presumptuous of Mr. Atlas to insert himself into Bellow’s world and, in effect, cast judgment on it.

“To write a biography of Saul Bellow would be, in a sense, to write my own autobiography, a generation removed,” Mr. Atlas wrote, noting that his parents grew up in the same Jewish immigrant neighborhood in Chicago where Bellow spent his youth.

Mr. Atlas was unsparing in his accounts of Bellow’s five marriages and multiple affairs, describing the women in the author’s life as “secondary figures who served his own fantasies of them as providers, entrappers, sexual predators: the Enemy.”

“It becomes clear,” critic James Wood wrote in the New Republic, “that Atlas is not writing the biography of a freedom-loving mind, of an imagination, but of a seducer, a bad husband, and money-earner who also happened to write some good books.”

While working on Bellow’s biography, Mr. Atlas was also a journalist and publishing executive. He developed the idea of publishing a series of short biographies written by notable authors, first under the Penguin Lives imprint and later for his own publishing company.

Mr. Atlas matched author and subject — Larry McMurtry writing about Crazy Horse, Christopher Hitchens on Thomas Jefferson, Bobbie Ann Mason on Elvis Presley — for biographies of about 150 pages each. The series of more than two dozen books sold hundreds of thousands of copies, with the improbable top seller being Garry Wills’s biography of St. Augustine.

“Writing a biography resembles writing a novel in that you have to solve the problems that present themselves in writing any narrative,” Mr. Atlas said in 2000 in an interview with Humanities, the journal of the National Endowment for the Humanities. “That is why I claim that biography is an art form, because it involves the same narrative and aesthetic questions. The only difference, I’m tempted to say, is that you’re dealing with fact instead of inventing facts.”

James Robert Atlas was born March 22, 1949, in Chicago and grew up in Evanston, Ill. His father was a book-loving doctor, his mother a homemaker.

Mr. Atlas graduated in 1971 from Harvard University, where he studied with poets Elizabeth Bishop and Robert Lowell, then spent two years as a Rhodes scholar at the University of Oxford. His mentor there was Richard Ellmann, the author of celebrated biographies of James Joyce and Oscar Wilde.

“Ellmann’s ‘Joyce’ didn’t read like a biography: it read like a work of art,” Mr. Atlas wrote in his 2017 book “The Shadow in the Garden: A Biographer’s Tale.” “It had the authority of great fiction; it was scholarly but not academic; and behind its facade of objectivity you could detect, if you listened closely enough, the biographer’s own voice. This was the kind of book I aspired to write.”

Mr. Atlas moved to New York, intent on becoming a major figure in the city’s literary life. He worked at high-profile publications, including Time magazine, the Atlantic, the New York Times, Vanity Fair and the New Yorker, while writing essays, reviews and his biographies of Schwartz and Bellow.

In 2005, he published a collection of essays, “My Life in the Middle Ages,” about the concerns of growing older, including losing his job at the New Yorker when he turned 50.

Survivors include his wife of 44 years, psychiatrist Anna Fels of New York; two children, Molly Atlas of New York and William Atlas of Los Angeles; a brother; and a grandson.

A new biography of Bellow’s early years, by literature professor Zachary Leader, appeared in 2015 and was “so claustrophobic,” Times critic Dwight Garner wrote, that he returned to Mr. Atlas’s book “with fresh eyes for its best qualities: its human scale, its economies of style, its sense of the sweep of Bellow’s life and its rich understanding of his milieus.”

When Mr. Atlas completed his biography, he said Bellow asked him, “So what have you learned?”

“I said, ‘I’ve learned that you can never know a person.’ ”