"It's an escape from doing other people's books, and moving back to doing my own books now," he says. "It's got much more of my opinions, my style, my view of the world, and is a much more personal book in the sense that no one has ever done anything like this before. But it's also a nod towards Agatha Christie and the Golden Age of crime fiction."
Along with Graham Norton's Holding and Sophie Hannah's second Hercule Poirot novel Closed Casket, Magpie Murders has been included in a revival of so-called "cosy crime," although Horowitz himself isn't fond of the term.
"Classic crime is how I would describe it, or genteel crime," he says. "It's something that I feel very comfortable writing, as I helped create Midsomer Murders all those years ago. I love the intellectual rigour of it and how every clue in Magpie Murders is presented fairly.
"When you get to the solution - or indeed the solutions - you realise that you have had the answers right in front of your eyes all the time. In fact, it's possible to solve a large chunk of Magpie Murders from the first page, but it's only when you get to the end that you realise why."
Magpie Murders actually dates back to the late 1990s when Horowitz was working as a writer on the first two seasons of the long-running ITV detective show.
"The idea first came to me nearly 20 years ago, and if you watch 'Written in Blood', the second episode of Midsomer Murders, you'll see that a character is actually reading a copy of Magpie Murders," he says. "I first conceived it back then but I've waited all this time to write it because I knew that it was going to be a very difficult book to write and I wasn't yet a good enough writer to write it."
While Horowitz insists he "wasn't trying to pastiche or imitate" Agatha Christie or other Golden Age crime authors such as New Zealand's Dame Ngaio Marsh, Atticus Pund nevertheless bears a close resemblance to Hercule Poirot.
He says it was difficult to create a detective that wasn't like Poirot or Sherlock Holmes. What Pund shares with Poirot is that he's also a refugee in Britain, although in Pund's case he's Greek-German.
"He was in the concentration camps during the Second World War, which has given him a very particular view of life, and what man is capable of doing to man. He's older than Poirot but there are remnants of Poirot in him, and by the time you get to the end you'll understand why. It's not coincidental, as it's something that's there by design."
Opening with a biography of fictional Atticus Pund author Alan Conway and even praise from the likes of Ian Rankin and Robert Harris for his supposed past works, Magpie Murders explores the "often oppressive" relationship between the writer, their fictional creation and the readers.
"Agatha Christie called Hercule Poirot 'a pompous little bore', and Sherlock Holmes was actually reviled by Arthur Conan Doyle, who at one stage had him thrown off the Reichenbach Falls," says Horowitz, who set his 2014 novel Moriarty in the immediate aftermath of the apparent demise of Holmes and his notorious arch-enemy.
"Doyle actually wanted to kill Holmes by the end of his career, and I find that very interesting because I created Alex Rider, and have written 11 books about him, and I sometimes get these feelings of hostility towards him," continues Horowitz, who will publish a new novel starring the teenage espionage agent next year.
"I sometimes end up resenting him because he's 14 years old, he's really cool, and he's having these great adventures while I'm sitting in a room getting old by myself. Kids love Alex Rider and have no idea of the issues and problems of this poor soul having to write about him in a flat in London."
Comparing it to a Russian doll, Horowitz believes that Magpie Murders operates on several different levels.
"The inner doll in the middle is the world of Atticus Pund, and the characters that live in the village where that part of the book is set, while the second doll is the supposed real world of Susan Ryland and the publishers," says Horowitz, who has also drawn on his own life.
"There's actually a third doll, which is my world, as every single location in the book has been deliberately chosen because it's connected to me in some way. It's like there are worlds within worlds, and the fun of the book is that through her investigations Susan Ryland realises that these worlds are connected in all sorts of ways, and of course, they're connected to my world as well."
Magpie Murders
by Anthony Horowitz
(Orion Publishing, $38)
