Shortly after discovering his grandfather’s writings, Puchner confronted his father, who haltingly confirmed Karl’s anti-Semitic thinking. He shared with Puchner a sweet family photo of a 1937 picnic enlarged to reveal swastika buttons on Karl’s jacket. But the discovery revealed an even greater shock.

It was “allowed to drift into the background,” Puchner writes in his blend of memoir and philology. “It was their mother who persuaded them to let the matter rest. ‘Leave the old man alone,’ she said about her husband, who was then 61 years old. There was no more talk about it at all,” an exchange emblematic of Germany’s postwar shame and denial.

Research took Puchner from Harvard’s great library back to Germany to uncover his Uncle Günther’s methodical attempts to resuscitate Rotwelsch. The coded mashup of German, Hebrew, and Yiddish was rich with words that reflected its speakers’ lives: on the fringes of society, on the run from the law. (Anti-Semites over the ages, including his grandfather and the theologist Martin Luther, made much of the connection between what was viewed as a criminal argot and Jewish culture.) Sore meant both stolen goods and trinkets for trade; there were many words for police, including greife — one who grabs.

Puchner, who has studied languages all of his professional life and can speak more than six, said Rotwelsch was the one he “took for granted.”

“The language is great fun and witty. I would sometimes regale my friends, ‘Have you ever heard of this?’” he said. “But then I learned about it as part of this itinerant underworld. It expresses the difficulty of this life, and police and prison. Many of these speakers found themselves on the road because of war and economic upheaval.”

Rotwelsch signs, or zinken, were visible for Puchner, who grew up in Nuremberg, as a marker on a street or at his house, where a cross with a circle around was an indicator that his mother would feed itinerants. Toward the end of the book, Puchner met, through an interpreter, a nomad named Chief, who described the glory of Rotwelsch as its ability to recycle words to preserve its very tradition. The Chief laughed at Puchner’s attempts to define the language by Western standards.

“‘You will never know this language,’ his laughter was saying, ‘because you live in another world, a false world … and my whole way of life, my entire language, is one big laughter with you which we, the kochemer, the ones in the know, make fun of you,’” Puchner wrote.

In Nuremberg’s archives, Puchner, who was raised in the city that had become a difficult part of Germany’s past, discovered his Granduncle Otto had served as a Nazi paramilitary brownshirt (along with Puchner’s grandfather), only to later become head archivist of the Nuremberg trials.

“I knew I had an uncle that was an archivist, but it’s an odd thing tied to the national reckoning. As children, we studied the Holocaust very thoroughly, but I didn’t connect it to my own family,” he said. “These people were central figures and not-so central figures who were still around in some powers. Most of the former Nazis stayed in power and rebuilt Germany after the war. It’s what my father and uncle’s generation rebelled against.”

Puchner, who became a U.S. citizen in 2007, thinks about Germany’s reckoning, especially as he has watched his adopted country grapple with its painful racist history this year.

“I think about what’s happening right now, with the changing of street names, and the tearing down of monuments and statues. Suddenly German history seems to resonate in a strange way with both our national reckoning and the fear of fascism,” he said. “What this book has taught me is that tearing down statues is not enough. Your own identity in all of that has to be on the line as well. What happened in Charlottesville with white supremacists waving Nazi flags and a German-American president who doesn’t distance himself from it, and German neo-Nazis waving confederate flags, and the QAnon movement — these are not just historical parallels. There are actual, strange connections between the two countries.”

In his own family, facing the past has been particularly difficult, with some relatives becoming defensive, even hostile.“Some no longer speak to me, which surprised me because I was super careful and wrote about living people only as much as was necessary,” he said. “But I learned from these reactions that, in a way, it doesn’t matter what you write, or how careful you are. The sheer fact that you write about your family upsets other members because it’s not their story.”

Puchner thinks of the book as “part of my inheritance,” though it has left him with just as many questions to wrestle with as answers he must accept.

“I’ve puzzled how my grandfather could believe these things, write these things, join this movement. Was he taken in by these conspiracy theories much like these QAnon ideas today, or was he more cynical? He was not from a very educated family, and maybe he wanted to break into academia. I could imagine asking these questions. I don’t know if he would know himself,” he said. “I wanted to come to some closure, though I wasn’t sure writing the book would do that, and I don’t think it has.”