It's an understatement to say that 59-year-old Horowitz is prolific. He has written dozens of books, including the much-loved Alex Rider series for children, and he is the screenwriter for television shows such as Midsomer Murders and Foyle's War. He published his first Sherlock Holmes homage, The House of Silk, in 2011. That took him about four months to write. Moriarty took a year.
"The book was written very, very carefully," he says. "I had to really think about almost every single sentence. My single rule was that the book had to be completely honest. It's a sort of sleight of hand. I've always had a great interest in magic. I love illusions. I don't do magic myself" - his fans would disagree - "but I love watching it."
The book gave Horowitz the opportunity to capture the mood, grime and atmosphere of 19th century London, including a brutal showdown in the tunnels beneath Smithfield meat market, which is near his home in Clerkenwell, central London.
"Smithfield, where the book climaxes, is two minutes from my door. It still operates but I wasn't able to visit the tunnels because they are closed and it's too dangerous. But it wasn't for want of trying. As for trying to capture 19th century London, since I was in my 20s, I have read 19th century fiction: Dickens, Trollope and, of course, Conan Doyle himself. So in a sense a lot of it is in my blood already. I love the sense of sitting at my desk and shutting my eyes and imagining the cobblestones and the fog and the hansom cabs. It's a wonderful world to escape into."
DI Jones was an interesting character for Horowitz to develop. Jones was the detective in Conan Doyle's second novel, The Sign Of Four, a case he badly botched. Dr Watson's account of the case - written as The Sign Of Four - publicly humiliated Jones and he is determined to prove his worth in this case. In fact, the whole of Scotland Yard is glad Holmes has gone. He made them look incompetent.
"I was really interested in the idea that if you were a detective in the time of Sherlock Holmes, you were in a fairly hopeless situation," says Horowitz. "You turn up at the scene of the crime, you ask questions - and you get it wrong. I can only think of one story where a detective gets it wrong and pulls one over Holmes, Wisteria Lodge, but detectives had this awful life. There is a chapter [in Moriarty] where all the detectives from every single Holmes story come together and realise they are better off without him.
"Jones is a Holmes wannabe," he adds. "He has based his whole life on this man. He admires him, he studies him, he wants to be him. He is smart and very clever, but not clever enough."
In the last Holmes book by Conan Doyle, The Final Problem, he mentioned a 15-year-old blond, slightly plump Swiss peasant boy who witnessed the struggle between the detective and Moriarty at the falls. Horowitz brings him back, as Perry, his favourite character in the novel. "In a funny way, he connects with my kids' writing for years. He is the same age as Alex Rider but he is absolutely a psychopathic, sadistic, malevolent, mad little boy. He was enormous fun to write. The idea is to give people a romp, I wanted it to be fun, build that energy into it."
On Monday night, Horowitz is talking at an event at the Museum of London, which has just opened a huge Sherlock Holmes exhibition. "It's quite interesting that the museum can run an exhibition about a man who didn't exist," he remarks. "But that just shows you the power of the creation, this extraordinary grip that this man has on our consciousness. It gives you a picture of 19th century London all hooked into this figment of imagination.
"Holmes has never been busier. You've got Benedict Cumberbatch who's about to return for a fourth season. The third Robert Downey film is in development. Ian McKellen is about to turn up as Holmes on TV in his great old age after his retirement, where he is a beekeeper, and Elementary [the TV series] is in America, so it is a rather crowded scene at the moment. People can't get enough of him. Part of his appeal is that he is the gentleman detective. He is like you or like me, but he is cleverer. He is the purist of all modern detectives."
Horowitz wrote Moriarty with a fountain pen, following Conan Doyle's method. These days he is channelling Ian Fleming, pounding away on a typewriter, writing a new James Bond novel at the invitation of the Fleming Estate.
"I'm well into it, it's going well. I was working on it last night, it doesn't leave me alone, it gets me writing all the time. It's much harder to write than Sherlock Holmes, much harder. Fleming's style is very much more difficult to capture and there are many more issues because although our attitudes towards Holmes haven't changed, our attitudes towards James Bond have. It has to be done carefully to be true to the original, true to Fleming's vision yet without offending people like yourself, who might have issues with some of the sexism, the snobbery, the racism, the homophobia."
Holmes has never been busier, says Horowitz, with Benedict Cumberbatch about to return for a fourth season, with Martin Freeman as the faithful Watson.
Well, William Boyd got it so wrong with his 2013 Bond saga, Solo, in which he committed the crime of making Bond seem rather camp. "I can promise you, you will not find my James Bond camp," Horowitz thunders. "I am keeping very close to the original, it is quite hard-edged and it has some really disturbing moments in it. Listen, let's talk in a year's time, ha ha ha."
As I am talking to Horowitz, I'm looking at a photo of him which was published in the Mail Online in 2011. The photo was taken when he was a sweet-looking 15-year-old boy at Rugby School, to accompany his story about his prep school, Orley Farm in Harrow, where he was beaten mercilessly by the headmaster and bullied by teachers he described as "sadists, perverts or alcoholics - and sometimes all three".
The only good thing to come of it, he wrote in the Mail, was that his misery and lack of confidence propelled him into a world of fantasy, of reading, writing and storytelling.
"That's absolutely true," he says. "The English education system back in the 60s was very strange. Wealthy children like me were sent to schools where we were so abused. I was not sexually abused but I think I was physically and mentally abused at that place.
"It was a horrible experience and not one that I was glad about in any way at all except for the fact that it drove me into reading. I had to find my escape and books became my lifeline. It was a big springboard into what I was going to do with my life.
"The miracle for me was that it didn't do more damage. I was very lucky that I met a wonderful woman when I was in my 20s [TV producer Jill Green], who I married and we had two very handsome young boys. That was the making of me but if she hadn't taken me on, I don't know what would have become of me. My parents died very young and I was never able to talk to them about why they sent me to this place but I know if my children had gone to a school where they were in hysterics when the beginning of the term was coming up and screaming and crying about not wanting to go back, I would have thought that maybe we should send them somewhere else."
Horowitz had a remote relationship with his father, a London financier who faced bankruptcy and hid all his assets in an unnumbered Swiss bank account, which was never traced after his death. But Horowitz does have something to thank him for. When he was 17, his dad gave him the complete set of Sherlock Holmes short stories and novellas.
"I was hooked. There is something in those stories that never leaves your system. I have read those stories two or three times over the course of my life and it is always like returning to an old friend. I would say that Sherlock Holmes is why I, with Foyle's War and Midsomer Murders and all the other writing I've done, it is why I started writing mysteries."
Moriarty (Orion Books $37.99) is out now.
