A soldier’s story: Telegraph journalist Cornelius Ryan interviewed 1,144 people for his book on the D-Day landings... Not that he was by any means from the gung-ho school of news reporting. Once the war was over, he spent many years compiling the material for The Longest Day, which was first published in 1959. In the course of his work, he interviewed 1,144 people.

“He wanted everyone’s story,” says archivist Doug McCabe, who supervises the Cornelius Ryan collection of archives at Ohio University, in the United States. “He took great pains to ensure that someone was where he said he was, and that someone else was there to confirm that story”...