HOW To Train Your Dragon author Cressida Cowell knows things are going well when she's sobbing in her writing shed, tucked away in her back garden.

"I deliberately set out to make myself cry," says the bestselling writer and illustrator. "I'm always making myself cry. Unless you can make yourself cry, how are you going to make other people cry?"

Penning the final book in her famous Dragon series, published 12 years after the first one hit the shelves, was particularly emotional. "I was in floods of tears," she recalls. "It was very hard to say goodbye. I loved writing about that world. People would say, 'You've written 12 books. Aren't you bored?' But I was never bored."

Before completing the Dragon series, which has sold eight million copies around the globe, Cowell, 52, began to think about what would come next. So she created her first new series in 18 years: The Wizards Of Once. Featuring the adventures of boy wizard Xar and girl warrior Wish, the first book was published last September to rave reviews. Twice Magic, the follow-up, is now hitting the shelves – and she admits the Dragon series was a tough act to follow.

"I was very anxious about writing Wizards," the married mother-of-three confides. "I didn't want to disappoint. It's quite hard when you've had a huge success. I've been so lucky but there's a certain pressure."

She need not have worried – the first in the trilogy was a hit and DreamWorks have snapped up film rights. The animation company previously adapted her Dragon books into a TV series and films, with the third movie, How To Train Your Dragon: The Hidden World, due out in February. Cowell will be jetting out to Hollywood to meet the film-makers, although she doesn't have creative control over the movies.

"Each film costs $150m to make and the same to market. That's a lot of money and a lot of jobs. I think it would be almost irresponsible of the film company to allow an author to have that," she reasons. "Having said that, I've had a great relationship with them and they've always sent me scripts. They haven't wanted to make a silly action movie about dragons. They've thought about what it's all about."

Despite her huge success, Cowell – who says she was "constantly in trouble" at school for being "hugely disorganised" – faces a certain amount of snobbery for being a children's author.

"People do sometimes say to me, 'Why aren't you writing for adults?' As if that's a grander, more laudable thing to do. I just don't understand it. Writing for children is just as challenging. They are interested in the essential things – death, life, heroism, philosophy."

She is passionate about getting children reading for pleasure, something she says is in danger of being lost, thanks in part to what she calls an "obsession with grades" and "literacy modules", and an absence of "long-term thinking" in today's education system.

"I did English at Oxford and I didn't have a single grammar lesson. There were no grammar lessons at school and I went to a very English pushy girls school. We read a lot and I was exposed to Shakespeare and Chaucer, but it was about reading for enjoyment. My spelling wasn't very good and neither was my handwriting, but a teacher gave me a book where I could just write stories."

Cowell's books are read by almost equal numbers of girls and boys. It was important that a female character, Wish, drives the action in her new series of novels.

"I love the Famous Five, but Anne was always going off and making the flippin' sandwiches when the adventure was getting interesting," she says of the Enid Blyton books.

The female protagonist in her new series is not 'beautiful', she adds: "It's heartbreaking when you see how much pressure there is on girls and how much they mind about it. It's so destructive."

Wizards Of Once is set in an ancient, magical time full of wizards, warriors, giants and sprites – where the wild woods are under threat. Cowell, also the author of the Emily Brown picture books, was born in London but spent much of her childhood on a small, uninhabited island off the west coast of Scotland, without a TV or electricity, where she drew and wrote stories.

"I worry about children's lack of contact with nature," she says. "Children need to be wild. When I was growing up, my parents opened the front door, booted you out, and said, 'Come back at lunchtime', and it didn't matter where you were."

  • The Wizards Of Once: Twice Magic Book 2 by Cressida Cowell (Hodder Children's Books, £12.99)