Sands was given access to this trove by one of the most intriguing central characters in “The Ratline”: Horst Wächter, the fourth of Otto and Charlotte’s six children, who had been safeguarding the papers in his crumbling Austrian schloss. Horst is an interesting case — forthright but also perversely myopic. He steadfastly refuses to acknowledge his father’s complicity in the Holocaust, in spite of all evidence to the contrary, including the evidence in documents in his possession. The tension between Sands and Horst, the questioner and the questioned, gives “The Ratline” much of its driving force. Sands had met Horst while researching “East West Street,” and the two men later became part of a documentary, “My Nazi Legacy,” in which Sands brought together Horst and Niklas Frank, the son of Hans Frank, the Nazi governor general of occupied Poland, who was tried and hanged at Nuremberg.

While Niklas, a journalist, is unforgiving of and unsparing about his father — he carries a photograph of his dead body, snapped in Nuremberg after his trial, “to make sure that he is really dead,” he tells Sands in “East West Street” — Horst, in sharp contrast, refuses to acknowledge his father’s actions, and prefers to see his father as a good man caught up in a bad system. He was especially close to his mother and believes his father was poisoned — a hypothesis Sands spends much of “The Ratline” pursuing. And yet for all his blindness, Horst has done a great service to history. Rather than destroying the documents in his possession, he let Sands scrutinize them, a move that put him at odds with members of his family, who didn’t want to call attention to their ugly past.

And ugly it was. The correspondence is a grotesque, intimate look at total commitment to Nazism, horrific evil interspersed with the banality of upper-middle-class life. In the summer of 1942, when the “Grosse Aktion,” or mass deportation of Galicia’s Jews, was well underway in territory Wächter governed, he and Charlotte moved into a lovely villa outside Lemberg, with a swimming pool and a tennis court. He entertained Himmler, and a close aide to Hitler praised Wächter’s skills to the Führer. Returning from a summer holiday in 1942, Wächter wrote to Charlotte: “Jews are being deported in increasing numbers. It’s hard to get powder for the tennis courts.”

Charlotte, who died in 1985, was utterly dedicated to her husband and to the Nazi cause. On the shelf in Horst’s castle, Sands found a copy of “Mein Kampf,” which she had inscribed to Otto: “Through struggle and love, to the finish.” When Otto was in hiding in Rome and in need of money, Charlotte sold off works of art she had looted from collections in Krakow. And after Otto’s death in Rome, Charlotte managed to transport his body back to Austria, illegally.

It’s a testament to Sands — his fiercely inquiring mind, his excellent researchers, the wealth of documents and his ability to make them come to life — that the book is so suspenseful. “The Ratline” was a podcast for the BBC before Sands put it into book form, and his style here is to bring us along on the quest. There are many extraordinary secondary characters and subplots. Rather than citing the work of scholars, he pops in for visits, including one with David I. Kertzer, whose excellent book “The Pope and Mussolini” offers a vivid picture of the Vatican during the 1930s.