WITH more than 20 million books sold worldwide in more than 30 languages, as well as numerous television deals and stage adaptations, there's no doubting Terry Deary is one of the most popular children's authors of all time. But what exactly is the secret to his huge success?

"There's no real secret at all," says Terry, speaking in his discernible Sunderland tones. "It's pretty simple actually. I'm a children's author, first and foremost, who was asked to write non-fiction. My strength is communicating stories about people, not boring facts and figures, and that's what makes these history books so different."

The 62-year-old author, who lives in County Durham with his wife Jenny and daughter Sara, adds: "There's nothing in non-fiction for youngsters that's even come close."

Now Terry has adapted two of his bestsellers, The Terrible Tudors and The Vile Victorians, for the stage. Produced by the Birmingham Stage Company, both shows arrive at Wycombe Swan next week.

The Terrible Tudors revolves around the life of Henry VIII and his "headless wives", while The Vile Victorians takes a tongue-in-cheek look at the filth of the 19th Century factories and the misery of the mines. It even brings the ill-fated Charge of The Light Brigade to life, with some spectacular 3D effects.

"I think we've really moved theatre forward with these shows," explains Terry, who worked professionally as an actor and director before turning to writing.

"It's amazing what we've been able to achieve. With the 3D graphics, it's almost as if there are cannon balls, sometimes even heads, that literally bounce into the audience."

Terry began writing the Horrible Histories series in 1993, beginning with The Terrible Tudors and The Awesome Egyptians. He had initially been asked to produce a "history joke book with a few facts," but the project soon turned in a different direction.

"Well, I was actually finding the facts far more interesting than the jokes, so I decided to turn it around," Terry tells me. "The result was something quite informative, but in an entertaining way.

"What I really didn't expect is that the series would be going 15 years on and still selling well. It really has been a phenomenon."

Popular with youngsters for their particularly gory details and descriptions, The Horrible Histories series has also been lauded by adults for getting children more interested in history.

For Terry, that comes something as a "sweet victory" against his own negative experience of education as a child.

He explains: "At school I found history very dry and my teachers certainly did a good job of squeezing out any love I may have had for the subject. Even today, everything taught in schools is reduced to testing, making it all very dry indeed.

"If I had it my way, I wouldn't have schools at all. They don't educate, they just keep kids off the streets. But my books educate, because they prepare kids for life."

He continues: "Some people have criticised my stories for being too graphic, but I think children are far less traumatised than adults by them, because they lack the imagination to know just how bad something is.

"I'm also not someone who's going to wrap children up in cotton wool. There's a real world out there and things they need to be aware of."

With an ironic tone, Terry adds: "Just take Henry VIII as an example. He's always portrayed in schools as this jolly, fat man, but in fact, in some ways he was a monstrous psychopath.

"It's outrageous - why don't we start telling children the truth about history? I hope my books do just that."

Aside from the stage adaptations, Terry has many more Horrible Histories in the pipeline, as well as plans for a television series. He's also embarked on turning some of the series into fiction and is currently working on The Blitz, a wartime story as seen through the eyes of two evacuees.

Horrible Histories: The Vile Victorians arrives on Thursday, May 29, Saturday, May 31, and Sunday, June 1; and The Terrible Tudors arrives on Friday, May 30, and Sunday, June 1, at the Wycombe Swan. Tickets: 01494 512000.