Murder most foul is afoot again as Anthony Horowitz’s quirky characters return in “Moonflower Murders” on PBS’ “Masterpiece Mystery” at 8 p.m. Sunday. A sequel to “Magpie Murders,” also based on a Horowitz novel, “Moonflower Murders” features Lesley Manville playing detective.

But her character, Susan Ryeland, is not any old sherlock. She’s a middle-aged editor turned hotelier who goes about solving murders in the six-part series with a literary sensibility.

Horowitz has a third book on the way.

“This one is called ‘The Marble Hall Murders,’” he says. “It is a variation on the theme, and the book has been extremely well received by my American publishers, my Canadian publishers and my British publishers, and, most importantly of all, by my wife.”

Jill Green, Horowitz’s wife of 36 years, is also the executive producer on the PBS shows.

“We started collaborating on ‘Foyle’s War.’ It was the first show we did together,” Green says. “When we’re working on these very complex shows, things change all the time and we have to think outside the box, sometimes very quickly. To have that tremendous work ethic is fantastic.”

The complexity she’s talking about is the fact that the narrative of the plot moves between two worlds: the real world of a book publisher and her alter-mentor, a peerless detective from a 1950s novel — a gumshoe that only she can see. This is easy enough in book form, but not as simple for television.

Even though these three novels are beleaguered by crime and his books — “The Word Is Murder,” “The Sentence Is Death,” “A Twist of a Knife” may sound profoundly deadly — the 69-year-old Horowitz insists his work is not dark.

“I think what I write is entertainment and very life-observing, but I guess that the darkness in them, the murders and the violence and the sense of threat, comes from my own childhood — those unhappy days between 8 and 13 that have never quite gone away.”

It’s the period when Horowitz was sent away to boarding school by his privileged parents.

“I was extremely unhappy there because if you were in English boarding school back in the 1960s you had to be one of two things: very clever or very athletic. And I was neither,” he recalls. “I was an oversized child. I wasn’t very bright. I wasn’t doing well in class and I didn’t have many friends.”

Then two things changed it all for him. The first was his discovery of the school library, where he found refuge in reading adventure stories, and absorbing and living them. He also discovered the ability to tell stories to other kids in the dormitory at night.

“I was 10 years old, and I knew I was going to be a riter and there was no Plan B.

There was definitely no Plan B. Horowitz has also scribed the Alex Rider series about a teenage James Bond, and he has dallied with Sherlock Holmes and written the addicting “Foyles War,” about an uncompromising small-town detective during World War II.

He says he is fascinated by writers who create great characters and then feel that they’re somehow beneath them.

“Arthur Conan Doyle created Sherlock Holmes, the greatest detective who ever lived, and dislikes him so much and feels he’s so far beneath him that he throws him off a waterfall at the Reichenbach Falls,” he says. “That is not true for me. Every character I’ve created, from Alex Rider to Hawthorne to all the characters in this show, I love. And I don’t have any sort of views of myself as being too good for this. I love murder-mystery writing.”