'The best question I ever got asked," says Charlie Higson, the beginnings of a rueful smile appearing on his face, "was by a kid.

He said 'I don't mean to be rude, but don't you think you could have done more with your life than write stories about zombies?'. At which point I had a bit of an existential crisis. I thought: 'He's right. This is no job for a grown man.'"

It's certainly true that zombies have occupied a good deal of Higson's working life in the half decade since he sat down to write The Enemy, the first in a series of novels set in a post-apocalyptic London. But while the comedian-turned-children's author may be no closer to answering that disarming question about the books' relative worth, the publication this week of a fifth instalment, The Fallen, does at least bring him closer to the end of the cycle. It has also brought him to Edinburgh from his home in London, and it's there we meet ahead of his appearance at the Edinburgh International Book Festival. Cue more pesky questions from young readers, no doubt, though at the moment Higson looks perfectly relaxed thanks mainly to the tan he's sporting from a recent holiday in Tuscany.

The Enemy series, as it's known, deals with the aftermath of a mystery illness which has turned all adults into zombies. They roam the streets in packs and are known as "sickos" by the children and teens who remain unaffected and whose lives (and loves) Higson charts when he's not setting up breathless chases and set-piece battles.

Originally intended as a trilogy, the sequence will now become a septet. That's if Higson stops there. He's resigned to the fact that public appetite among his mainly teenage audience may demand further instalments. "The Fallen was originally meant to be the second book in the series, but then I went off at a tangent," he explains when I ask how three became seven. "It was an accident, partly. When I wrote The Enemy I was intending the second one to just carry on, but I had a slight problem with that."

The problem was this: at the end of The Enemy, the children who are its focus split into two groups. One goes to the Tower of London and one heads for the Natural History Museum. "I realised I was going to have to introduce two whole new sets of characters and I was worried the second book would get bogged down in that and there wouldn't be enough story to move it forward. So I thought about writing a second book to set up the other groups of characters."

That second book spawned a third and a fourth and, as a result, he says, "it's gone a bit Game Of Thrones in terms of its ever-expanding cast list". Not content with that, Higson has also imposed a complex chronology on the series. Second novel, The Dead, takes place "just over a year before the events described in The Enemy", according to its frontispiece. Book three, The Fear, begins five days before the end of The Dead and book four, The Sacrifice, eight days after the end of The Fear. Got that?

Essentially what it means is that they all precede the first book. Except they don't really: as the action moves forward, the stories catch up with each other, so that at points they become contemporaneous and parallel. Higson keeps most of it in his head. His publishers prefer a highly detailed flowchart.

The Fallen, then, reunites readers with Blue and Maxie, hero and heroine of The Enemy.

"They aren't in three of the books of the series and if I'd taken a more conventional approach they would have been," says Higson. "But I quite like doing things this way. You obviously have to please your readers, but the idea of Hollywood-style test screenings and rewriting everything to please an anodyne, central plateau - well, you make things considerably less interesting if you give people what they think they want."

The structure and the fractured chronology also allow the same events to be seen from different viewpoints. It's what Higson calls "the classic Rashomon approach" after Akira Kurosawa's 1950 film (remade in 1964 as The Outrage, with Paul Newman), which uses that device. "I like that idea of stories intersecting. You can build in good surprises and twists which keep the more on-the-ball reader entertained."

One of the most pleasing aspects of the novels is Higson's appropriation of famous London landmarks. Along with the Tower of London and the Natural History Museum, Buckingham Palace and the Palace of Westminster are cleansed of their former inhabitants. Given the Higson treatment, they become instead sanctuaries, headquarters or battlegrounds for communities of children. Those of a pedagogical bent can view the children's responses to the buildings as a prism through which to view their pre-zombie function. Everyone else can just enjoy the image of near-feral kids running amok in the House of Commons and the Royal Mews.

"I was obsessed with places like the Natural History Museum when I was a kid and I'd fantasise about going and living there," he says. "They'd have these diorama, like a caveman dioarma where there'd be a nice hut in a glass case, and I always thought 'Wouldn't it be fun to get inside?'".

Fantasy is one thing. Actually doing feet-on-the-ground research is another, but Higson has been well-served by the gatekeepers of London's tourist attractions. A question posted on Twitter asking if anyone knew how to set up a behind-the-scenes tour of the Tower of London, for instance, elicited an immediate response. "By the afternoon it was all set up. And that's happened on two or three occasions. I had a fantastic day at the Natural History Museum - rooms and rooms of stuff that you never ordinarily see. You get so many ideas when you're looking around a place in person."

Even Buckingham Palace opened its doors. Sort of, anyway. Invited to an event celebrating children's literature ("a load of authors and children, and a big, terrible stage show which had the cast of Harry Potter in it"), Higson took the opportunity to tour the grounds and as much of the building as he could talk his way into. That turned out to be a substantial amount. So as JK Rowling chatted to minor royals at a barbecue, Higson was being driven round in a golf buggy in the company of Anthony Horowitz and Horrid Henry creator Francesca Simon. It's quite a picture.

But after five years working on the Enemy novels, there is a sense that Charlie Higson is coming up for air. Down The Line, the radio satire he writes and performs with friend and long-time collaborator Paul Whitehouse, recently finished a fifth series on BBC Radio Four, and he's currently co-starring in new Jack Docherty-penned sitcom Start/Stop on the same station.

He even found time to write an episode of Miss Marple which aired earlier in the year. "I had a sudden flurry of stuff on the back of that which I said yes to in the hope that most of it won't actually happen," he laughs. "One is to try to develop a big, early evening Saturday ITV family drama in the Dr Who/Merlin sort of slot - a big fantasy adventure."

Whether Blue, Maxie, the sickos and the rest can be put aside long enough to write it remains to be seen.

The Fallen is published on by Penguin, £12.99