It must be a nervy business being Robert Harris' publisher. On the one hand, you have an international brand-name thriller writer whose book sales exceed 10 million. On the other, you never quite know what he's going to deliver next. Many authors would have been forgiven for milking Fatherland - his best-selling, what-if debut novel - for a few Nazi sequels, but in the subsequent 20 years Harris has turned his hand to code-breaking (Enigma), modern Russia (Archangel), ancient Rome (Pompeii, Lustrum), Tony Blair (The Ghost) and a very 21st-century financial meltdown (The Fear Index).
So it comes as no surprise to discover that his latest novel takes readers off in a completely new direction, this time to late 19th century Paris. An Officer And A Spy retells the Dreyfus affair, one of history's great miscarriages of justice and a true story that resonates with modern parallels (think secret trials and Guantanamo).
It's told through the eyes of Colonel Georges Picquart of the French Army, who realises that his fellow officer, Alfred Dreyfus, a Jew, has been falsely accused of leaking secrets to the Germans.
When Dreyfus is convicted of treason and sentenced to solitary confinement on Devil's Island, off the coast of South America, Picquart must decide whether to take on the army, intelligence services and government to prove his innocence. It's a question of duty and principle faced by every whistleblower down the ages: follow his conscience or be loyal to his employers.
"Over lunch in Paris two years ago, I happened to ask Roman Polanski [who directed the film of The Ghost] if he had ever considered doing the Dreyfus affair," Harris, 56, tells me at his Victorian vicarage in West Berkshire. "I had seen some books in his office and he said he'd always wanted to do it, but he had never been able to find a story. I said I'd take a look. And almost from the moment I started reading about Dreyfus, I saw that at the heart of the whole thing is a brilliant spy story, which has tended to be lost in all the social commentary about anti-Semitism."
It's also a compelling tale of power, cover-ups and idealism - meat and drink for Harris, a former political journalist. In some ways, the only surprise is that he hasn't turned to the subject before. As far back as 2001, he was comparing the treatment of his close friend Peter Mandelson, who had just been sacked for the second time by Tony Blair, to the Dreyfus affair.
"The thing I like doing is writing about power and structures and how they affect life. In this case you really see a model of how bureaucracies in any society and at any time cover up their mistakes, and how they will square this in their own conscience by saying that it's for the greater good. I found writing the book quite radicalising in a way. I don't think I will ever look at the government and armed forces and institutions generally in the same way again."
Harris was helped in his research by the French Government's recent decision to make available online all the secret files relating to the case (affairedreyfus.com). At the rotten heart of the conspiracy against Dreyfus was an intelligence unit called the Statistical Section, located in crumbling offices behind the Ministry of War.
"A bit like MI5 in the early 70s, the Statistical Section was going rogue and made up of a group of quite paranoid right-wing figures. When you write historical books, you bring what you know about the present to it. And I'm sure that little set-up [the Statistical Section] was like MI5, seeing traitors everywhere, breaking into things, forging things, generally quite out of control."
The figure of Picquart - cool, aloof, highly intelligent - provided Harris with his introduction to a large cast of characters that includes ministers of war, generals, spooks, lawyers, gay diplomats, forgers and handwriting experts. Intriguingly, there was little love lost between Picquart and the man whose freedom he was fighting for. "I love that understatement in their relationship. These two men were utterly entwined and dependent on each other, and yet there was no contact between them."
Harris says that if he had been writing a screenplay, he might have been tempted to up the feelgood factor and bring the two men closer together. But he had agreed with Polanski that he would do the novel first and work on a film afterwards.
Harris wrote an acclaimed screenplay for The Ghost, but he was recently fired from the film of his last thriller, The Fear Index, something that clearly still rankles.
"I had fulfilled my contractual obligation, which was to write the first draft. Then I was told my services were no longer required. It did finally confirm to me that the reason one writes novels and one has chosen this life is to be captain of one's own ship. I will never put myself in that position again.
"The moment anyone can pitch in and the power is no longer yours, you are a hired hand and my entire life has been a desire to escape that. The biggest benefit by far of making money by writing has been independence. I can get up and write what I want in the day and no one will stop me."
Harris rises at 4.30am when he's working on a novel, and his writing day is finished by lunchtime. He researched this book for a year and began writing in mid-January. "I like writing in the first six months of the year because once you're really into the book, the light seems to follow. The days get longer."
He writes 1000 words a day, after editing the previous day's work. There is no second draft. "When I've finished the book, I go over it for a week or 10 days, but broadly speaking, that's it."
Only once has he suffered from writer's block, when he was struggling with his second novel, Enigma. He eventually found a way through, thanks to the letters of Raymond Chandler.
"He said writing wasn't easy but he couldn't understand how anyone could be a proper writer and complain about this incredible privilege and joy of inventing words. And it powerfully affected me because I had got into a syndrome of melancholy about writing.
Ever since then, I've never complained about being a writer, even in the darkest days when a book is in trouble. I get a pleasure out of the struggle now."
An Officer and a Spy (Hutchinson $37.99) is out now.