"I didn't want to jump on any bandwagon and I'm not particularly fond of that very cynical form of publishing, where you take a famous book and either re-write it or do a sequel or a prequel. The honest truth is that some of those are less good than others. It's quite an interesting job and there's a thin line between writing a homage, a pastiche or a rip-off. I just wanted to write an exciting, interesting novel, which was very much in the spirit of the originals."
Having first read classic Holmes novels like A Study In Scarlet and The Sign Of Four when he was 17, it was Horowitz's long-standing love of the character that convinced him to take on the challenge. "There's something very strange about Sherlock Holmes, especially if you're an English schoolboy," he recalls. "When you read the stories, they stay with you forever. I often think that one of the reasons I took up murder mysteries in so much of my writing is that I was inspired by Conan Doyle to start with and later, by Agatha Christie and Dorothy L. Sayers."
Horowitz says there are parallels between Conan Doyle's classic canon and his own stories. "All my life I've been writing murder mysteries, whether it's Foyle's War, Midsomer Murders or Poirot," he says. "My writing has always been what you call narrative fiction in the sense that it's got very strong plots and twists at the end. In the end, it was a very good fit and it helped that I knew Sherlock Holmes inside out."
Rather than his Alex Rider or The Power Of Five series, Horowitz believes it was his small-screen adaptations of Agatha Christie's best-selling whodunnits that gave him the ideal background to tackle Holmes.
"When I wrote Poirot, I had to write not just in Agatha Christie's voice but also for David Suchet, who plays Poirot," he says. "My very first job on television was writing Robin Of Sherwood, which had been created by Richard Carpenter. My job was to write like him, which is what I did. What I've done with The House Of Silk is to become invisible, so even though my name is on the cover - and it's my work, so why not? - when you read the book it should really feel like it's written by Conan Doyle."
Unlike Sherlock's 21st century reboot, Horowitz has resisted bringing Holmes into the present day. "It's written very much in Watson's voice and I was careful to use words that weren't modern," says Horowitz who, as in the majority of Conan Doyle's books, has told The House Of Silk from the loyal doctor's perspective.
"But what makes it modern and hopefully attractive to a modern and young audience is the speed of the story-telling, the chases and the twists at the end. I was aware from the start that this had to be what you call an airport or a holiday read so it has to be fun and not something you struggle through to work out the syntax. I try to hook people and there are lots of surprises in the book. What pleases me is that nobody who has read it yet has guessed the ending, which to me is the ultimate test."
Beginning with an unpublished manuscript being discovered at a London solicitor's, Horowitz imagines The House Of Silk - and interweaving second tale The Man In The Flat Cap - as a missing case that was too horrific to tell in its day.
"There has to be a reason why Watson didn't include it with all the original stories, so there has to be something so unpleasant, shocking, scandalous and dangerous about it that he bundled it up and put it into a vault," says Horowitz, whose novel is around twice the length of Conan Doyle's longest, The Hound Of The Baskervilles. "My first challenge was how to stretch the elements and simplicity of a Conan Doyle Sherlock Holmes novel into a 90,000 word format. I basically came up with two plots but wrote them in such a way that they became inextricably linked, as Watson puts it in the book."
Along with the final Power Of Five installment, Horowitz is penning the script for the second Peter Jackson-directed Tintin film. "His studio has an amazing set-up for motion capture and animation," says Horowitz, who recently visited New Zealand. "Weta is just mind-boggling."
The House Of Silk (Orion $34.99) is out now.