There are high-achieving workaholics - and there is Anthony Horowitz. Not only did the 56-year-old Londoner have a hand in writing some of the most popular British television shows of the past 25 years, among them Midsomer Murders and Agatha Christie's Poirot, he also created Foyle's War, a crime drama set in the Second World War - a show with such colossal grass-roots appeal that the eruption of anger after ITV summarily axed it in 2007 meant it was recommissioned, pronto.
Of Horowitz's 50-plus books for adults and children, the best known is the Alex Rider series about a 14-year-old boy spy. Naturally, when the time came for Stormbreaker, the first title of the nine, to be filmed in 2006, it was Horowitz who wrote the screenplay. He's a popular storyteller in the Arthur Conan Doyle tradition and, as it happens, a big fan not just of Sherlock Holmes but of other Conan Doyle creations such as Professor Challenger and Brigadier Gerard. So it was clever of Conan Doyle's literary estate to pick him to do its controversial bidding: write a new Holmes novel, which it would endorse as "official" - in other words, admit to the canon of 60 tales.
The result, The House of Silk, has been justly acclaimed as intelligent, respectful and beautifully written. Horowitz catches Dr Watson's tone completely in what amounts to a remarkable feat of literary ventriloquism. It's easy to believe that it dates from the 1890s as purported and is of a piece with the other stories that Conan Doyle wrote between 1887 and 1927.
"I'd been a fan of Holmes since I was about 16," Horowitz explains, "but writing a Holmes novel wasn't something I'd thought about doing. After the Doyle estate approached me, the first thing I did was reread all the stories. I found they were still familiar to me - somehow they stay with you for life.
"Then I wrote five pages of the book: the first five pages, just to see if I could get the language right. I realised that I could, but that's only because it's so brilliantly done in the original. If you read the stories, if you immerse yourself in them, Watson's voice is so distinctive - so extraordinary and precise and warm - that actually you only have to listen and out it comes again."
He says the Conan Doyle estate left him to it, at his request. "They didn't look over my shoulder. I made it clear that I wouldn't take notes from them or show them work in progress. I wanted to write it as if it were my own novel."
Presumably they're pleased with the result, though? "I don't know. I haven't heard."
Horowitz is currently working on the screenplay for the second of Steven Spielberg's Tintin films, based on Hergé's book Prisoners of the Sun. The first, just out, seems to have pleased the average punter but alienated some devoted Hergé fans such as the novelist Tom McCarthy, the author of Tintin and the Secret of Literature, who attacked it in the press.
"I read those pieces," Horowitz admits. "But you know, the film has been very well received in [Hergé's native] Belgium. There's always going to be a problem when someone adapts or continues a series like Tintin or James Bond."
He must have been nervous about The House of Silk's reception?
"You don't write for the fans, but at the same time, if you can avoid annoying them, then obviously that's better. Because I so admire Conan Doyle … I wanted to write the sort of book he would have been pleased to see. That was my main goal. Pleasing the fans is secondary."
Still, The House of Silk is full of in-jokes that only fans will get, such as the revelation of Inspector Lestrade's first name and Watson's disapproval of the late-period Holmes story The Adventure of the Blanched Soldier, which Holmes narrates himself. Horowitz had particular fun with a scene in which Holmes deduces that Watson's wife Mary is away nursing a child who has flu.
So successful is The House of Silk, you finish it hoping Horowitz will find time to write another. But he doesn't want to: "I don't think I could better this one, though I would like to go back and explore that Victorian world again with different characters."