One day in late 1978, the historian Robert H. Ferrell drove eight hours from Bloomington, Ind., to the Harry S. Truman Presidential Library and Museum in Independence, Mo., in search of letters the 33rd president had written while serving in World War I.
His disappointment at not finding much about Truman’s Army service was quickly allayed when the chief archivist suggested that Mr. Ferrell look through boxes of newly released materials containing the handwritten diaries Truman kept during and after his presidency.
The diaries, letters and memorandums Mr. Ferrell found derailed him from his research into a book about President Woodrow Wilson and World War I to edit and annotate “Off the Record: The Private Papers of Harry S. Truman” (1980). From then on, he became the consummate Truman scholar, writing or editing 11 more books about him.
“My dad appreciated his small-town sensibility and his love of books,” his daughter, Carolyn Ferrell, said in a telephone interview. “And my dad truly believed something Truman said: ‘There is nothing new in the world except the history you do not know.’ ”
A longtime history professor at Indiana University, Mr. Ferrell died on Aug. 8 at a nursing facility in Chelsea, Mich., near his home in Ann Arbor. He was 97. His daughter confirmed the death.
Over six decades, Mr. Ferrell wrote or edited more than 60 books, largely about American diplomacy, war and presidents. He had an abiding fascinating with World War I — in which his father fought — and with the diaries of statesmen and soldiers. But once initiated into Truman’s world, Mr. Ferrell kept returning. He spent so much time at the Truman library that he rented an apartment in Independence.
“He was a diligent and careful man who had a great eye for new materials,” said Samuel Rushay, the library’s supervisory archivist.
Mr. Ferrell was at the library in 1983 when an archivist alerted him to a newly opened trove of more than 1,000 letters Truman had written between 1910, when he met his future wife, Bess Wallace, and 1959. In Truman’s hand, the letters provided a record of their devoted marriage and his life as a farmer, haberdasher, county judge in Missouri, United States senator from Missouri, vice president and president.
Mr. Ferrell quickly read through the archive, selected several hundred of the letters and wrote the accompanying text to “Dear Bess.” Published in the summer of 1983, it became a best seller.
“Forgive me, but what we have here is an American classic,” Mr. Ferrell told The St. Louis Post-Dispatch shortly after the book’s publication. “Mr. Truman was a most remarkable man. And he was a good writer, mainly because he was direct and had a sense of humor.”
Truman gave his wife a ringside seat to the Allies’ conference in Potsdam, Germany, in the summer of 1945. In a letter written on July 18, with the war in Japan still being fought, he wrote that Stalin had “moved to make me the presiding officer,” which he said “made presiding over the Senate seem tame.”
“The boys,” he added, “say I gave them an earful.”
But more important, he wrote, he had “gotten what I came for — Stalin goes to war on August 15 with no strings attached to it.”
Mr. Ferrell believed that Truman needed Stalin’s help to end the war in the Pacific quickly, even though he knew that the United States had successfully tested the atomic bomb in the New Mexico desert on July 16. He wrote that “Truman did not think American force alone, conventional or nuclear” could beat the Japanese.
But the nuclear bombs detonated over Hiroshima on Aug. 6 and Nagasaki on Aug. 9 led to Japan’s surrender soon after.
Robert Hugh Ferrell was born on May 8, 1921, in Cleveland. His father, Ernest, was a banker, and his mother, Edna (Rentsch) Ferrell, was a schoolteacher.
A talented pianist, he studied music and education at Bowling Green State University in Ohio, but his education was interrupted by World War II. He served as a chaplain’s assistant in the Army Air Forces before being promoted to staff sergeant, with duties that ranged from preparing payrolls to helping move his squadron to new forward bases in Europe.
He returned to Bowling Green after the war to finish his degree, but he was no longer interested in teaching music. Transformed by the war, he began to read the work of historians like Arthur M. Schlesinger Sr., Allan Nevins and Ida Tarbell. He received a master’s and a Ph.D. in history from Yale, where he was an acolyte of Samuel Flagg Bemis, an expert on the early diplomatic history of the United States.
“From Bemis, he learned that to be a history professor you have to be writing history as well,” William Pickett, a former student of Mr. Ferrell’s at Indiana University and a professor emeritus of history at Rose-Hulman Institute of Technology in Terre Haute, Ind., said in a telephone interview.
Mr. Ferrell expanded his dissertation into a book, “Peace in Their Time: The Origins of the Kellogg-Briand Pact” (1952), an examination of the failed international agreement of 1928 to prevent a second world war, which won the American Historical Association’s George Louis Beer Prize. He followed that with a book about President Herbert Hoover’s foreign policy and a biography of Senator Frank B. Kellogg, the Minnesota Republican who was an author of the Kellogg-Briand Pact.
He began teaching at Michigan State University in 1952 and the next year joined the faculty of Indiana University, where he stayed until retiring in 1988 to focus on writing books — always on a typewriter, while consulting notes he had written on index cards.
“He had also had enough problems with politics at Indiana,” his daughter said. “He made a lot of enemies.”
By 1994, Mr. Ferrell had completed four Truman books, as well as the one about President Wilson and World War I that he had put aside years earlier, and a textbook on American diplomacy. He also edited the wartime and presidential diaries of Dwight D. Eisenhower.
That year, building on his Truman scholarship, Mr. Ferrell published “Harry S. Truman: A Life.” It came out two years after a better-known biography, David McCullough’s “Truman,” which won a Pulitzer Prize and had sales that overshadowed Mr. Ferrell’s book.
In its review, Kirkus Reviews wrote that while Mr. Ferrell’s book “cannot begin to compete” with Mr. McCullough’s narrative verve, “Ferrell partly makes up for this with his mastery of Truman sources” and “his shrewd analysis of the workings of executive power.”
The prolific Mr. Ferrell also published a book that year about the Democratic National Convention of 1944, at which Truman was chosen as President Franklin D. Roosevelt’s running mate.
He was not yet done with Truman. In 2002, he edited a book of Truman’s writings, “The Autobiography of Harry S. Truman,” which included his commentary. Four years later he wrote “Harry S. Truman and the Cold War Revisionists,” a series of essays about his postwar foreign policy.
Mr. Ferrell continued to write well into his 80s. He published his final book, about an all-black Army regiment in World War I, in 2011.
In addition to his daughter, Mr. Ferrell is survived by two granddaughters. His wife, Lila (Sprout) Ferrell, died in 2002.
Clifton Truman Daniel, one of Truman’s grandsons, said that Mr. Ferrell’s deep knowledge of his family was at first daunting.
“The first time I worked with Bob Ferrell at the Truman library nearly a quarter-century ago, I was a nervous wreck because he knew more about my family than I did,” he said in an email. “The nerves didn’t last. He was easygoing, interested, engaged and full of good humor.
“I don’t think I’ll ever know as much about the Truman presidency as Bob did.”