My book The Happy Traitor began more than 20 years ago when I chanced upon an article in a Dutch magazine. It was by a radio journalist who had interviewed George Blake, a British Dutchman who had started out spying for the British and wound up a KGB double agent. I’d never heard of Blake before, but was struck by the similarities in our backgrounds: we’re both British, Jewish (half, in his case), cosmopolitan, and we grew up in the Netherlands within 20 miles of each other.

Blake had had a fascinating life: from Dutch Resistance to captivity in North Korea to spying for MI6 in Berlin while secretly working for the Soviets, getting the longest jail sentence in modern British history, then legging it from Wormwood Scrubs to Moscow, where he ended up watching communism collapse. And the amazing thing to me, reading the article in 1999, was that this relic of history was still alive and able to reflect on it all.

I decided I wanted to interview him, too. The story of how I got to him is in the book, but suffice to say that one morning in May 2012 I found myself walking into the roomy garden of his dacha outside Moscow. We spent about three hours together, and given that he was a de facto serial killer, who had sent perhaps 40 agents of the UK to their deaths, we got on frighteningly well.

I’d planned just to write up the story for a Dutch newspaper, but I walked out of the dacha afterwards thinking, “That was the most interesting interview I’ve ever done.” There was more here than just a newspaper article.

At the time my kids were small, and I was spending every weekend almost entirely in the playground. I had no hope of finding time to write a book, and I’d never written about espionage before, so I pushed the project to the back of my mind. But I found that I remained obsessed with Blake (no disgrace – so was John le Carré), and I began reading what literature there was about him.

By about 2015, my children were getting older, and I got the odd moment to myself at weekends. I began writing Blake’s biography. I thought that the value I could add, besides having spent those hours with him, was the international perspective: Blake had previously been written about by British writers, using mostly British sources, but that approach doesn’t work well with a cosmopolitan subject. I wanted to understand him using Dutch, German, French and Russian sources. (A friend helped me with the Russian.)

The archives of the Stasi, the East German secret police, were a particular treasure. Blake gave four speeches to the Stasi between about 1976 and 1980, and these had been filmed. The Stasi, as was its wont, had also done its own research on him, and all this was in the files. The great thing about the Stasi for researchers is that the state it serves went defunct. MI6 has never released its file on Blake, and the KGB files closed down again after a brief phase of openness in the early 1990s, but since the collapse of the GDR, the Stasi archives have been opened to the public. Helpful archivists sent me all the material I requested for a small fee.

Blake had given me the interview on condition that I not publish it in a British newspaper. His ex-wife and sons in Britain, conservative types, didn’t like publicity around their KGB family member. But the mutual friend who had brokered our interview later gave me his blessing to publish the book after Blake’s death. Then, he said, the family would have to deal with a wave of publicity whether I wrote anything or not. Clearly my decision to publish was in part selfish. But I also felt that Blake owed the British public an explanation.

The hardest part of the research was getting my head around the nuances of espionage, a field in which almost nothing is as it seems. What has pleased me most about the reception of the book is that reviewers with expertise in intelligence have taken The Happy Traitor seriously, even if some of them feel I’ve been too kind to Blake.

I know writers are supposed to say that every book is a struggle – like suffering from a long and painful illness, said Orwell – but I found Blake so fascinating that it rarely felt like work.

The Happy Traitor. Spies, Lies and Exile in Russia: The Extraordinary Story of George Blake by Simon Kuper is available now. You can read more about George Blake from Simon in Issue 4 of our magazine, with subscriptions available here.