For most of his papacy, Benedict has spoken little about his past.
During much of the Third Reich, he lived with his family in the small, Catholic town of Traunstein, Germany, not far from the part of Austria where Hitler was born.
"So much of that colored almost every aspect of his life," said Bunson. "He grew up within the sound of church bells in the valleys. He went to mass on Sundays. His entire family life was Catholic and he spoke of the beauty of the church liturgies and the mass."
His father, who was in local government service, resisted the Nazis and, as a result, the Ratzingers kept a "low profile," according to Bunson, who has plans to write another biography when Benedict's successor is known. No one in the family ever joined the Nazi party.
"He understood very quickly that he was asked to do things he could not in his conscience do," he said. "They had to move because of pressure to conform to the Nazi party and the whole ideology of the period. That, combined with his own Catholic experience, made him very opposed to the ideology of the Nazis."
Ratzinger entered a preparatory seminary for the priesthood in 1939, but joined the Hitler Youth at the age of 14 when it became mandatory in 1941.
"He wrote about the disgust he had -- the exercises they had to go through and the expectation placed upon them," Bunson said of the Hitler Youth.
He and his fellow seminarians were drafted into the Nazi's anti-aircraft corps in 1943. A year later, he was drafted into the regular military, then was sent home. He was called up once more, but deserted in 1945, and was captured by U.S. soldiers and held for several months as a prisoner of war. He was only 18 when the war ended.
Young Ratzinger had already "heard the call of the priesthood," according to Bunson.
After Ratzinger deserted the military, he hid to escape the Gestapo units which were "rounding up deserters and shooting or hanging them," he said. Fortunately, he was captured by the American forces and spent time in an internment camp in southern Germany.
"What is interesting is that this was his first experience dealing with Americans and he later wrote about that," said Bunson. "He found Americans very easy to get along with, very impressed with them and, in a way, it laid the groundwork for his high opinion of the United States."
Bunson noted that Catholic priests were also condemned to concentration camps and many worked to hide Jews.
But in 1998, Pope John Paul II apologized for some Catholic participation in the Holocaust, which set the stage for closer ties between the church and the Jewish community.
"Then Cardinal Ratzinger was one of the most warmly received elections in recent years from many in the Jewish community," he said. "He's such a known quantity to them. He has traveled extensively and is friends with many Jewish leaders around the world." . Benedict's legacy included two achievements "worthy of mention" that Jewish groups applauded and confirmed his standing as a friend, according to Hier of the Wiesenthal Center.
On Dec. 3, 1993 John Paul II recognized the state of Israel, a move that would not have been possible without a "nod" from Benedict, then the cardinal in charge of theology, according to Hier.
"When John Paul II broke with a 2000-year tradition of the church, that Jews would be eternally punished for their refusal to recognize Christ and it would therefore be impossible for the state of Israel to be born, he was in direct contravention to punishment," said Hier. "He had to get the nod from Ratzinger who was the major theologian and at the time in charge of Catholic doctrine."
"Ratzinger said that the score would be settled in the end of time," said Hier. "This was very important."
In his memoir, Benedict also dispelled traditional thinking that Jews were complicit in the death of Christ.
"That has the imprimatur of a pope," said Hier. "One hundred years from now, a pope who wrote a book on the subject will be an important achievement in Catholic-Jewish relations."