Daniel Aaron, Harvard professor emeritus, in his office in 1985. (Mark Lennihan/Associated Press for the Washington Post)

Daniel Aaron, a literary scholar and historian who helped develop the multidisciplinary academic field of American studies and who helped launch the publishing project of literary classics known as the Library of America, died April 30 at a hospital in Cambridge, Mass. He was 103.

The cause was complications from pneumonia, said a son, poet Jonathan Aaron.

Dr. Aaron, the son of Jewish immigrants from Russia, became one of the foremost scholars of American culture and helped shape a new field of academic study and codify the nation’s literary legacy.

He was a professor at Smith College and later at Harvard University and was the author or editor of dozens of books. His scholarly interests included the effect of the Civil War on literature, leftist writers of the 20th century and the 17 million-word diary of an eccentric misanthrope.

In a career spanning much of the 20th century, Dr. Aaron came to know many of the seminal authors and poets of his time and of earlier generations, including William Butler Yeats, Sinclair Lewis, Robert Frost, Lillian Hellman, Truman Capote, Mary McCarthy and Ralph Ellison.

Daniel Aaron in 1985. (Mark Hannihan/Associated Press for the Washington Post)

In Dr. Aaron’s youth, the study of American culture was not yet a widely accepted topic of study in the ivory tower. He went on to receive the first doctorate awarded by Harvard in “American civilization.”

Literary scholar Helen Vendler pronounced Dr. Aaron “a chief founder of the discipline of American studies” in a 2012 interview with Harvard magazine, adding that he “advocated the scholarly study of American authors at a time when universities still emphasized English and European literature.”

Although he was nominally a professor of English, Dr. Aaron designed eclectic courses that combined literature, history, politics, sociology, anthropology, art history and other fields. Hundreds of universities now offer American studies programs.

“The notion was that everything was your province,” he told the Boston Globe in 2001. “If you’re studying the society — a civilization, to give it a grand name — everything was germane.”

From 1979 to 1985, Dr. Aaron was the founding president of the Library of America, a publishing project inspired by a French series of books of classic literature and championed by literary critic Edmund Wilson.

Dr. Aaron helped define the ambitious agenda of the Library of America, which Washington Post book critic Jonathan Yardley called “our literary equivalent of baseball’s Hall of Fame.”

The first book in the project, which now includes more than 250 volumes, was a collection of three lesser-known novels by one of Dr. Aaron’s favorite writers, Herman Melville, the author of “Moby-Dick.”

Beginning in the late 1940s, Dr. Aaron often traveled abroad on behalf of the U.S. Information Agency as a visiting scholar in cultural exchange programs. He gave seminars on American life, including topics such as the arts, racial relations and humor.

“We probably made some of those we reached think better of Americans,” he wrote in “The Americanist,” his 2007 memoir, “and confirmed the opinions of others that the United States had no culture that Europeans need take very seriously, that America was rich and powerful but unlettered and parochial.”

To his surprise, Dr. Aaron later discovered that some of the people he encountered overseas — including members of the U.S. State Department — assumed he was a spy. When a journalist in Uruguay charged him with being a “cultural imperialist,” Dr. Aaron explained in “The Americanist” that “I had come to Uruguay to talk about North American literature, just as the Leningrad Philharmonic [then concertizing in Montevideo] had presumably come to play music.”

Among his later projects, Dr. Aaron spent more than six years editing “The Inman Diary: A Public and Private Confession,” a two-volume, 1,600-page opus published in 1985. The book was drawn from the private journals of an aristocratic Southerner named Arthur Crew Inman, who settled in Boston.

A wealthy hypochondriac who professed to be allergic to noise and sunlight, Inman lived from 1895 until he committed suicide in 1963. He wrote 17 million words in a diary he kept for almost 45 years.

A man of grotesque racial and ethnic prejudices, Inman nevertheless resolved to record his life with unflinching honesty. After meeting one woman, he wrote: “She is homely as a stump fence built in the dark; but she doesn’t giggle all the time.”

That woman became his wife.

Inman rode around Boston in a chauffeured 1919 Cadillac and paid strangers to visit his darkened rooms to tell him about their lives.

“He’d ask if they believed in God, if they took drugs, what their sex lives were like, whether they loved their wives or husbands,” Dr. Aaron told The Post in 1985.

Inman flirted with the women and sometimes seduced them.

Dr. Aaron became so immersed in Inman’s bizarre world that he wrote letters to the long-dead diarist, expressing both disgust and a dark fascination.

“The detestable sick soul had his mitigating decencies,” Dr. Aaron wrote in “The Americanist.”

“The Inman Diary” became an unexpected success and was reprinted several times by the Harvard University Press before Dr. Aaron published a one-volume abridgment in 1996, “From a Darkened Room: The Inman Diary.”

The diaries have been the basis of a play, an opera and a feature film, “Hypergraphia,” which is in production.

“I became stuck, absorbed, caught up in it,” Dr. Aaron told Time magazine in 1985 about Inman and his diaries. “I got to know him and his world in a way I know of nothing else, no other society.”

Daniel Baruch Aaron was born Aug. 4, 1912, in Chicago. He was 5 when his family moved to Hollywood, where his father, a lawyer, worked in the film industry.

By the time young Daniel was 10, his mother had died of tuberculosis and his father of multiple sclerosis. He, two brothers and two sisters moved back to Chicago to live with relatives.

After graduating from the University of Michigan in 1933, Dr. Aaron began graduate studies at Harvard, but, as a Jewish student, he didn’t feel entirely welcome. He finally found his niche in 1936, when the university inaugurated a new graduate program combining American history and literature. He received a doctorate in 1943.

“The United States suddenly loomed as the last democratic bastion in the world after the German occupation of France in 1940,” Dr. Aaron wrote in his memoir. “About then, I began to feel that it might be almost as important to understand American civilization as to preserve it.”

While in graduate school, he taught Harvard underclassmen, including future president John F. Kennedy, who wrote a “so-so examination paper for a Harvard American literature course.”

At Smith College in Northampton, Mass., where Dr. Aaron taught from 1939 to 1971, his students included Betty Friedan, who cited him in the acknowledgments of her groundbreaking book “The Feminine Mystique” in 1963.

Dr. Aaron joined the Harvard faculty in 1971 and formally retired from teaching in 1983. For years afterward, he continued to go to his cluttered office every day.

Dr. Aaron’s books included “Men of Good Hope: A Story of American Progressives” (1951), “Writers on the Left” (1961) and a popular textbook of U.S. history, which he wrote with other scholars.

In one of his most influential books, “The Unwritten War: American Writers and the Civil War” (1973), Dr. Aaron argued that many writers ignored or suppressed the moral implications of slavery as a cause of the Civil War and often portrayed African Americans as objects of “contempt or dread” or as “an uncomfortable reminder of abandoned obligations.”

As a result, Dr. Aaron suggested, racial injustice festered for a century after the Civil War before exploding in the 1960s. Writing in The Post, historian Steven Channing called “The Unwritten War” a “brilliant and disturbing book.”

Dr. Aaron’s 1964 essay “The Hyphenate Writer and American Letters” addressed the divided social identity of many Americans from ethnic and racial minorities. It is considered a formative document in the analysis of American life through the lens of multiculturalism.

Dr. Aaron, who received the 2010 National Humanities Medal, was a fellow of the American Academy of Arts and Sciences and a member of the American Academy of Arts and Letters.

His wife of 66 years, the former Janet Summers, died in 2003. Survivors include three sons, Jonathan Aaron and Paul Aaron, both of Cambridge, and James Aaron of Shutesbury, Mass.; a granddaughter; and two great-grandchildren.

Although Dr. Aaron was seen as a founder of American studies, he was never quite certain of his place in American life.

Never forgetting that he was a Jewish orphan who came of age in the 1920s and ’30s, he described himself in his memoir as a “native son neither estranged from the collective American family nor unreservedly clasped to its bosom.”