For almost six decades, it was missing, all 400-plus revealing, handwritten pages. The diary of Alfred Rosenberg, a Nazi theorist whose views on race are thought to have helped incite Hitler’s persecution of Jews, vanished after its author was convicted of war crimes and hanged at Nuremberg in 1946.

It would take two men — a former F.B.I. agent who specialized in recovering stolen art and the former chief archivist at the United States Holocaust Memorial Museum in Washington — a dozen years to track down the elusive artifact, one of few known diaries by a member of Hitler’s inner circle.

Working with federal investigators, they found it in 2013, in the offices of a publishing house in upstate New York, a place far more modest than the Bavarian castle atop a hill, Kloster Banz, where it had been first discovered in a vault after the war.

The recovery of the diary, now held by the Holocaust Museum, was announced three years ago. But the story of the hunt for it, of the tip that finally panned out, of the undercover work to track it and the crucial role played by Homeland Security investigators and a federal subpoena, is now being discussed in a new book, “The Devil’s Diary: Alfred Rosenberg and the Stolen Secrets of the Third Reich” (Harper), by the former F.B.I. agent, Robert K. Wittman, and the journalist David Kinney.