But one story did suggest that, given the chance to pursue the best material, Ryan could produce memorable work. In 1956, the liner Andrea Doria collided with a Swedish ship off the coast of Nantucket and sank. Ryan set about reconstructing the collision, the rescue of all but forty-six of the ship’s 1,706 passengers, and most memorably, the drama of a husband and wife who had switched beds the night before, only to be woken when a beam split their cabin—separating them, as it turns out, forever. The writing was at times overdone. But the reporting, which included the surviving husband’s moment-by-moment account of his wife’s demise, was a harbinger of the big projects to come.

Ryan had initially proposed a D-Day book about only the first two or three hours of the invasion. But then he began to report, and his ads (“Personal: Were You There on 6 June 1944?”) yielded thousands of responses. He followed up with a three-page questionnaire that could serve as a primer for reconstructing a narrative: Where did you land and at what time? What was the trip like during the crossing? Do you remember, for example, any conversations you had or how you passed the time? Were you wounded? Do you remember what it was like—that is, do you remember whether you felt any pain or were you so surprised that you felt nothing?

One thousand, one hundred, and fifty people wrote back. And of that group, he interviewed, alone or with his assistants, 172. Ryan’s daughter, Victoria Bida, told me that her father had once been away for eighteen months reporting, suggesting that to find the man, the reporter, you need look no further than his files. And to read the files—to deconstruct how the book was assembled, to connect names and stories in the book with questionnaires, interviews, letters, diaries, and regimental histories—is to feel yourself in the presence, so many years later, of a man compelled to learn everything.

Here, for instance, was the questionnaire from Lieutenant Donald Anderson of the 29th Infantry Division, who wrote about getting shot: “No pain, just stunned. Figured my brains were spilled all over my helmet.” Here was Ryan’s interview with General Maxwell Taylor, who commanded the 101st Airborne Division and who told him what it had been like parachuting into a dark field: “Lonesome as hell.” And here was the interview with Private Aloysius Damski, a Pole who had been forced to join the German 716th Infantry Division, who told of playing a card game called “scat” on the night before the invasion, then peeling away from his unit so that he could surrender to the British.

Then there was the material on Rommel, who committed suicide in October 1944 after he was implicated in a plot to murder Hitler. Ryan had the general’s diaries (nary an entry without comment about his dog) and an interview with his widow and son. But it was his adjutant, Captain Hellmuth Lang, who proved to be an interviewer’s dream. Lang recalled all the many telling details of the morning before the invasion, when Rommel, after a breakfast of tea and a slice of white bread with butter and honey, set out at precisely 6:47 a.m. in a black convertible Horch for his home in Herrlingen, where he would celebrate his wife’s birthday before continuing on for his meeting with Hitler. Frau Rommel later produced the birthday-gift shoes, long since resoled. And Lang, bless him, recalled the size: five and a half.

But Ryan was not only hunting for the small bits. As it happened, the Germans wanted it known that Rommel was not with his troops on D-Day because he was with the Fuhrer. Not so, Lang told him. He was at home—a discovery that was as thrilling as it was frustrating. Now he would have to rewrite the first chapter, and was already feeling overwhelmed by the task of culling, cataloguing, and deciding how best to use all the material he was gathering. “I do not know how I’m going to do this right now,” he wrote his wife Kathryn, a novelist who had also been his most valued aide.

Michael Shapiro is a contributing editor to CJR and teaches at Columbia's Graduate School of Journalism. His most recent book is Bottom of the Ninth: Branch Rickey, Casey Stengel, and the Daring Scheme to Save Baseball From Itself.