It takes a few anxious minutes for me to ring on Nick Harkaway’s north London doorbell, on the grounds that neither the taxi driver nor I is convinced it’s a private residence; its blankly businesslike doorway and proximity to a pub and an undertaker would lead a passerby to think of a discreet accountancy firm or therapist’s office. Or, perhaps, a safe house. But I double check the address and, seconds later, there is the proprietor, ushering me in. Two hours later, I’ve discovered that there’s more to the history of the Hungarian tobacco industry than meets the eye, that Harkaway tracks the shipping traffic in the Danube corridor via an app and that, despite not being a horsey person, he has a surprisingly detailed knowledge of how the late Queen’s horses were trained. And I’ve also got a little nearer to understanding how he came to take on one of the most daunting literary challenges imaginable: continuing the legacy of his father, John le Carré. The result is Karla’s Choice, a novel revolving around the celebrated mole hunter George Smiley and his enemy, the Soviet spymaster of the title. Set in 1963, it occupies the gap in the fictional chronology between The Spy Who Came in from the Cold and the trilogy of Tinker Tailor Soldier Spy, The Honourable Schoolboy and Smiley’s People, all published in the 1970s. As well as Smiley and Karla themselves, a host of figures from le Carré’s novels and their TV and film adaptations are there: Peter Guillam, Bill Haydon, Toby Esterhase, Connie Sachs and Control, the Circus chief whom Harkaway thinks of as “a monster – a spider in the web”. It’s a formidable undertaking, so how did it come about? When le Carré died at the end of 2020 at the age of 89, Harkaway explains, he left a letter of wishes to accompany his will, and asked that his family make efforts to secure and broaden his readership now and in the future. Those efforts may include adaptations, already a success – in 2010 Harkaway’s elder brothers Simon and Stephen founded The Ink Factory, the production company responsible for the TV adaptation of The Night Manager, and The Pigeon Tunnel, the documentary based on le Carré’s memoir. But another way would be to provide more books, “and so that conversation began, and I sat there very quietly and said, ‘Well, you know, there are lots of very talented people we should get to do this. We could get very interesting voices from around the world to write into different locations, because the cold war wasn’t just in western Europe. From a creative and literary viewpoint, those are absolutely fascinating projects.’ And then there was a pause, and my brother Simon said something like, ‘Well, I mean, there’s one person who very obviously should write it, and that’s you.’” It was an idea that Harkaway had previously ruled out. “I had said to myself, ‘I’m not doing this’ – not in a kind of horrified or aggressive way, but just like, no: I’ve spent however long, 16 years or something, putting clear water between myself and my dad in terms of creative work, and I’ve been very successful at doing that. You know, people occasionally find out and go, ‘God, I had no idea. That’s extraordinary. How does that work?’” Indeed Harkaway’s novels, from 2008’s The Gone-Away World through to Gnomon in 2017 and his most recent, Titanium Noir, are fuelled by fantasy and futurism. Despite their interest in espionage, surveillance and the digital world, they tend towards the absurd, moving easily, as Harkaway points out, from loop quantum gravity to jam and scones. That kind of thing, he says with bravura understatement, can’t happen in a George Smiley novel. But something about being asked by his family rather than suggesting it himself took the hubris out of the idea, and despite “the eye-watering fear”, Harkaway took himself off for a couple of weeks and “noodled around” to find out if he could do it. “And very quickly, I realised that I could. That you don’t have to turn the distortion dial on my normal writing voice very far before you get to something that feels quite comfortable in my father’s universe. And when I think about that, it’s obvious. I was born in 1972, when he was writing Tinker Tailor Soldier Spy. He was reading pages of it to my mother in bed in the mornings while I was learning to speak. It’s not just that I’m familiar with the work; it’s that the work is pivotal in how I learned to speak English.” In the introduction to Karla’s Choice, Harkaway recalls a childhood falling asleep listening to cassette tapes of his father’s Smiley novels read by actor Michael Jayston, who played Peter Guillam in the 1979 TV series in which Smiley was immortalised by Alec Guinness. It’s a striking, touching image, and I ask him more about it. “Picture an upstairs room with really 70s yellow bedroom walls with that slightly plucked texture, single-glazed, wood-framed windows facing due south, at one of the most southwesterly points in the UK, which gets hit by wind off the Atlantic Ocean at maximum heft. And the sound it makes when it gets stormy, whistling and howling around the house.” He listened to the tapes so much, even after he’d left the family home in Cornwall, that they became touchstones, comfort blankets. Were they also, I suggest to him, a way of taking his father with him wherever he went? “By that stage, not even that. When I was littler, definitely, there was a lot of hero worship going on, and there was a tussle in me because Dad read Smiley’s People, and I loved listening to Dad but I loved Tinker Tailor, and I loved Michael Jayston. And also listening to Dad on tape when he was down the corridor felt weird.” Harkaway is the youngest of le Carré’s sons: his elder brothers, Simon, Stephen, and Timothy who edited his father’s letters and died in 2022, were the children of le Carré’s first marriage to Ann Sharp. The couple divorced in 1971, and the following year he married Jane Eustace, an editor who took on much behind-the-scenes work in his writing career, and who died a few weeks after him. So Harkaway grew up both as the youngest son and, in a sense, as an only child, and his bond with his father was close: “a really easy relationship, not complex or difficult and he was proud of me and delighted with what I was doing”. Did he read his son’s books? Harkaway bursts into laughter. “Well. Now. Did he?” He considers. “He would always say to me, ‘I dipped into it, I haven’t really read it properly.’ And I knew that the things I write about were not really his thing. I was talking to my older brother a few years ago and said, ‘I’m not sure he ever properly read one of my books in its entirety’, and he said, ‘Well, I never had a conversation with him about any of your books where he didn’t know exactly what the plotline was, and most of the minutiae of the book.” I remark on the oddness of the inversion: people generally claim to have read books they haven’t, rather than the other way around. Harkaway puts it down to his father’s “extraordinary delicacy” in not wanting to squash or “bigfoot” him, and his delight when his son’s work was well received. And yet there were moments: Harkaway recalls a Cambridge don telling le Carré how much he loved his work, and also his son’s. “And he was very thrown. It was like, the ankle-biters have come for me. I said, are you joking? That’s one person, and your audience is millions of people around the world. My mother thought it was hilarious. She brought it up for weeks.” There are obvious technical challenges in adding narratives to the Smiley universe, and Harkaway was determined to avoid writing “infill stories”, and to create something substantial. One change he made was to picture George in a period of marital contentment, unlike later, when his wife Ann’s affair with Bill Haydon – among others – causes his notable distress. “I wanted to take that iconic sorrow,” says Harkaway, “and just breathe a degree of happiness into it – so that when it’s sorrowful, it really hurts.” Karla of course was key: a character whom Smiley first encounters in an interrogation room in 1955 but who, at that point, is only one of a number of agents he’s encouraging to defect. By the time we meet him properly in Tinker Tailor, Harkaway notes, Karla has become “the figure that Smiley is concerned about, and reciprocally, Smiley is the only person in the Circus that Karla thinks is dangerous. How does that happen? You’ve got a 10-year gap into which some momentous sequence has to occur to bring you to that point.” Why does he think his father left that gap? “Because he wasn’t building a franchise. He was just writing books as they occurred to him.” Aside from the imaginative demands of stepping into his father’s shoes, how did Harkaway cope with the emotions that the task must have brought up? After all, he was writing in the midst of loss and grief for both his parents. “Do you know, the weird thing about that is that it was, I think, really helpful. Because when you lose someone you love, you think the world should stop. You think the earth should crack and the heavens should rattle. And by and large, it doesn’t. Things just carry on. But when my father died, for an enormous number of people, the heavens did rattle. People cared, and they wrote and said that they cared. They said, ‘I didn’t know your dad, but his work meant so much to me.’ I had to go on the radio and talk about him, I think we did it in my parents’ living room. It was my therapy.” Harkaway is aware, naturally, of his father’s complexities – not least, his lifelong recovery from a childhood in which his mother left the family when he was five, and he and his brother Tony were brought up by his father, Ronnie Cornwell, a charming conman prone to occasional violence. (“Killing him was an early preoccupation of mine,” wrote le Carré, “and it has endured off and on even after his death. Probably it is no more than my exasperation that I could absolutely never pin him down.”) In 2023, Adam Sisman published a follow-up to his monumental 2015 biography. The Secret Life of John le Carré detailed the writer’s serial adultery, Sisman arguing that the author’s death had freed him to tell the whole story. How can that have felt for his family? “Broadly speaking,” Harkaway replies carefully, “we have, as a family, said what we need to say about that. I don’t see the world any differently. I think it’s just what it is.” His father died during lockdown, and there were strict limits on how many people could be present. With his mother unable to go because she was undergoing chemotherapy, the brothers discussed how to decide which of them should visit their father in hospital. “And I looked at my life and my life with dad, and I thought, ‘I do not have unfinished business. He loves me. I love him. We both know it. I don’t have anything pressing that I need to say except I love you, and he knows that.’ And so Tim went in. And then I think the following day, he deteriorated, and he died, and I didn’t get the chance to go in. Do I wish I could have held his hand? Yes. Do I regret the decision? Not for one second, and that feeling hasn’t changed.” His family and early readers have greeted Karla’s Choice enthusiastically, and with that “the level of fear declines, because at a certain threshold, no one can say this was a catastrophe, this is a disgrace. They can only say this is not for me, and everyone’s at liberty to say this is not for me.” If the wider readership agrees, Harkaway will continue to write in his father’s world, and his own too; spies in one direction, wizards and superheroes in the other. “The nice thing about writing into the Smiley universe,” he tells me as we part, “is that there’s a ceiling on how crazy I can be.” And so perhaps I am leaving a safe house. Karla’s Choice: A John le Carré Novel by Nick Harkaway, published by Viking. To support the Guardian and the Observer order your copy at guardianbookshop.com. Delivery charges may apply.