"It makes me realise that what I wrote was wrong," he reflects, "because success isn't like that when you're living it. In my book, the guy's novel becomes a best-seller and he takes off on holiday. I haven't had a holiday for two years because I've been promoting this book!" He laughs. "Though some small details have happened to me too: the posters in the Metro, the display devoted to him in his old school."
Dicker admits he has become so recognisable in his native Geneva that he can't have conversations on his mobile phone in public places and his girlfriend - a sports psychologist for the Geneva hockey team - has taken a while to get used to it. Since September 2012, when the book was first published in French, his life has been, in his own description, "a crazy whirlwind".
I first heard of The Truth About the Harry Quebert Affair at the 2012 Frankfurt Book Fair.
It is rare, these days, for a single book to generate widespread whispers of excitement.
But in October 2012, "the French novel with the long title" was genuinely the talk of the town. Everywhere you went, people would mention this book, sometimes pulling a folded piece of paper from their pockets to remind themselves of the name.
The book's central plot concerns a murder investigation in New Hampshire, reopened 33
years after the events in question when two bodies are dug up in someone's backyard.
That backyard happens to belong to Harry Quebert, a much-loved novelist in his 60s who
is still famous for a single book. Inevitably, the locals in the town of Aurora turn against him and he is arrested. The only person who retains his faith in him is Quebert's former student, starry young novelist Marcus Goldman, now crippled with writer's block.
Goldman sets out to solve the mystery, and the result becomes his second book.
It's like Twin Peaks meets Atonement meets In Cold Blood, with a bromance between literary jocks and some suspected paedophilia thrown in. It's about fame and infamy, writing and love, theft and imposture; about murder, madness, and religious zealotry. It's about guilt: not just in the criminal sense, but as an emotion that can dog you for life. It is breathtakingly plotted. But the fiendishness of the book's construction is not merely mathematical; it relies on the built-in ambivalence of each character, or there wouldn't be enough left to withhold for so long. The case looks to be solved several times, yet 100 pages before the end (of a 700-page total), just when all loose ends appear to have been tied, a sudden spinning ambiguity erupts in the clues. And we're still a good couple of resolutions away from the truth.
Publisher Christopher MacLehose, who has overseen Sam Taylor's seamless and fluid translation, describes The Truth About the Harry Quebert Affair as "a kind of reader's dream. I hadn't read a book as cleverly contrived as that for a very long time. People are going to measure other books by what he has done here."
Is it a French book, or is it an American book? And now that anyone can read the novel in English, does it make any difference? Dicker has been quite relaxed about the translation, and says he was happy to let the editors of the American edition (published by Penguin) change things they thought rang false.
"It's your territory," he told them. But MacLehose advised him to be cautious. "He said, 'it's a French book, set in the United States'." And that may be part of its success: Dicker is simply telling a story, not trying to be something else. The French edition contains no American jargon or slang. When asked if he was inspired by American film or television, Dicker admits that he had not even seen Twin Peaks until so many people compared his book to it that he was forced to find out what they were talking about.
Dicker tries to make light of his strangely peripatetic life. There are only, he says, "minor annoyances within a much greater joy", and wherever he is, there is always Skype.
"My day-to-day life hasn't really changed," he says, meaning that his friends are the same and his girlfriend is the same and he doesn't live on a metaphorical red carpet. But surely now, I suggest, there is no day-to-day life? He smiles, and gives up. "No," he repeats quietly, "There is no day-to-day life."
The Truth About the Harry Quebert Affair (MacLehose Press $37.99) is out now.