‘What is a ‘no-brainer’?” the German novelist Jenny Erpenbeck asks translator Michael Hofmann. It is the morning after they have jointly been awarded the International Booker prize, and Hofmann has just used the expression to describe the judges’ decision to award this year’s prize to Erpenbeck’s latest novel Kairos, which Hofmann translated.

But the novel wasn’t such a no-brainer back home, where, despite Erpenbeck being one of Germany’s most celebrated novelists, Kairos didn’t appear on the shortlists of either of the country’s most prestigious literary prizes. Now Erpenbeck has become the first German author to win the International Booker since it was established nearly a decade ago, putting the German publishing world into a bit of a spin. “They are kind of panicking,” she laughs.

The 57-year-old has written only four novels, beginning with her acclaimed debut Visitation in 2010 (all have been translated into English). Yet her name routinely comes up around the time of the Nobel, and the New York Times recently described her as “among the most sophisticated and powerful novelists we have”. Her second novel, The End of Days, about a Jewish woman born in Galicia in 1900, won the Independent foreign fiction prize in 2015. Her third, Go, Went, Gone, which tells the stories of a group of African refugees in Berlin, was longlisted for the International Booker in 2017. Her work also includes short stories, plays and nonfiction.

Hofmann is similarly one of Germany’s most influential literary figures: a poet and critic (notorious for his elegant hatchet jobs), as well as a prodigious translator. But this is the first time he has worked with Erpenbeck. “I’ve mostly been the translator of German 20th-century classics,” he says. On first reading Kairos he found it “a completely amazing book, irresistible. Irresistible, and also unpredictable.”

Kairos recounts a tempestuous (today it would be called “toxic”) affair between a 19-year-old woman, Katherina, and a married writer and radio presenter, Hans, who is 34 years her senior. Most of the slim novel is set in East Berlin between 1988 and 1992, though the story is bookended by contemporary scenes that explode everything in between. “On the one hand it is a love story, on the other hand it’s a political story,” Erpenbeck explains. “I tried to let them speak to each other, to interweave the historic events with a private problematic love story.”

“If you want a very short version,” Hofmann interjects. “It’s a book about duress.”

Set against the dying days of the German communist dream, the affair which begins so ardently becomes abusive, descending into violence, cruelty and surveillance as Hans becomes increasingly jealous and controlling. It is no surprise that the novel has been widely taken to be an allegory of the demise of East Germany. “I think the word allegory is a catastrophe for the book because you think one party is going to be one thing and one party is something else,” Hofmann says. “A more accurate way of describing it would be more like the interpenetration of personal and political.”

Erpenbeck started writing the novel after the noise of the 30th anniversary of the fall of the Berlin Wall in 1989 had died down. “What you try as a writer is always to get out of sight, to have your peaceful writing,” she says. “Writing has a lot to do with emotion, and also personal history. Reflection is a process, it’s not done quickly. If you write about something, you look at things in a different way than when you are just experiencing them.”

Erpenbeck’s project, over a career spanning 25 years, has been to cast new light on the last century or so of German history. In Kairos, she wanted to give voice to the East German people whose country was lost. “In the English-speaking world people are very interested to get a deeper insight into the stories; a need to understand better what happened,” she says.

“Even in Germany the East German stories are under-represented,” Hofmann agrees. “There’s a sort of feeling that we’ve done the east. The east isn’t interesting. I’m East German myself and that feels like nonsense to me. Kairos is a book out of its time. It takes time to assimilate things, to understand them, to be ready to talk about them.”

Initially Erpenbeck set out to write a book about the era that didn’t include the Stasi – although it inevitably creeps in. “I thought the only things that people know about East Germany is there was the Stasi and there was a wall. And that’s it. Because they saw The Lives of Others and more or less this is all they know. I wanted to add something to the picture.”

She also had to overcome her resistance to writing a love story. “It’s kind of a challenge. But I think I’m old enough now to write a love story because I’m out of danger,” she laughs.

Hans was a boy during the second world war, of an age to be part of the Hitler Youth, whereas Katherina is in her early 20s when the Wall comes down. Like the author, who was born in 1967, Katherina grew up in a tower block a stone’s throw from the wall, and shares other biographical details with Erpenbeck. While there has been speculation in Germany that the character of Hans is based on a real-life figure, Erpenbeck is keen to stress that he is a composite of men she read about during her research. “There were so many Stasi files revealed after the fall of the Wall, you can take anyone,” is all she will say now.

“The idea was to write about my experience of the time,” the author says. “It was high time to tell my and my friends’ stories. Many friends were kind of relieved, they had the feeling that they were seen in the book.” She wanted to capture how things looked and felt, details such as how the streets of East Berlin “immediately began to smell like Chanel No 5”. Her stories may be intimately personal but her characters cannot escape their history.

After the wall came down, her “childhood became a museum”, Erpenbeck has written. “I love museums, I love exhibitions, I love things being put together and commenting on each other,” she says now. She has been collecting ephemera from the GDR for many years, and one wall in her apartment in Berlin is covered in old wrappings, shopping lists and food cartons. She calls the novel “a museum in the form of a book”.

Unity, oppression, independence and freedom – the parallels between the romantic story and its historical backdrop emerged as Erpenbeck was writing. But, like the limbs in the novel, it is hard to disentangle them. “I just want to write a story that is alive,” the author says. There is definitely a racing pulse. Anyone who inhaled Milan Kundera’s The Unbearable Lightness of Being as a teenager will be reminded of his Prague Spring classic. Hans is charismatic and looks sexy with a cigarette (this is the 80s). Katharina is young and beautiful. From the start, there are enough red flags to alert the modern reader, and that’s before he gets the whips out. But as Erpenbeck points out, the balance of power shifts throughout the novel.

Hans represents history, while Katherina is the future. “She is starting out at the point when they meet. They fall in love and they are sharing a moment,” she says. “But then it turns out that she’s looking forward and he’s looking backwards. In the end, the problem is that she is still growing. She’s motion. And he’s trying to fix things and to keep her as she was in the beginning. But of course he cannot keep her in the same state as when they met.”

The intensity of the lovers’ relationship is captured in Erpenbeck’s highly charged prose, the story told in an urgent, immersive present tense from each of their perspectives, their thoughts running together, often with barely a line break between them. Like Irish novelist Paul Lynch, who won the Booker prize last year for Prophet Song, Erpenbeck has a laissez-faire attitude to speech marks – an approach that suited Hofmann’s sensibilities as a translator. “The reader has to bring their punctuation. The reader has to come with a bundle of inverted commas,” he says. “You know what somebody is thinking and not saying, or saying and not thinking. It keeps you on your toes. There’s something slightly infantile about needing these marks.”

Kairos captures the trauma and ambivalence of reunification. “The book goes on after the party is over,” as Hofmann puts it. One interesting advantage she has as someone from East Germany “is that we know both worlds, a socialist one and a capitalist one,” she says. “One aspect of the book is to have this comparison between two sides.” In the final pages Katharina is repulsed by the rampant consumerism of the west: “the much vaunted freedom of choice is a new can of worms.” Erpenbeck felt it important, she says, “to make clear how strange this world was to us. We never used to see beggars on the street, we didn’t have prostitution, we didn’t have sex shops, it was all completely new to us.”

“I try to make people aware that almost nothing is only black or white. Things need a more careful look, a more complex view to be understood,” she says. “I try to enable people to take a point of view that they haven’t taken before, so they can embody someone they are not familiar with, to understand better how things might be felt.”

Erpenbeck herself spent the evening of 9 November 1989 drinking wine and hanging out with her girlfriends. “And that was it.” They didn’t think to turn on the TV. The famous Bornholmer Bridge, which gave access to the west, was just around the corner. “If we had gone out, perhaps we would have run into all the millions who came. But we didn’t go out and we were on the fourth floor of a building, so there was no idea of any historic event going on.”

• Kairos by Jenny Erpenbeck, translated by Michael Hofmann, is published by Granta (£9.99). To support the Guardian and Observer, order your copy at guardianbookshop.com