The name Hoffman will forever be associated with tales of magic and enchantment, given the German Romantic storyteller and author of The Nutcracker. Over here, we have Mary Hoffman, whose career includes Amazing Grace (the picture book about a little black girl with big ambitions, which Laura Bush claimed as her all-time favourite) and the Stravaganza series, about teenagers who can time-travel to a gorgeous, violent version of Italy called Talia.

“The opposite of an overnight success,” as she puts it, Hoffman is one of many British children's authors who perfect their craft over decades, and when her historical detective story The Falconer's Knot was shortlisted for the Guardian Children's Prize last year it came as long-overdue recognition for an author who is finally hitting her stride as an important children's novelist.

At any rate, the woman who started writing children's books in 1970 at the age of 25, has the kind of tenacity that too many publishers tend to overlook, and at an age when many writers start to slow down is speeding up, now that her three daughters (one of them Rhiannon Lassiter, the SF and fantasy writer) are grown-up.

“My problem is I'm too versatile,” she says. “I've written more than 90 books, but in different genres, so it's been quite difficult to make a name for myself.” The child of a railway worker, she grew up in modest circumstances in South London, going to a state primary school where a beloved teacher encouraged her to apply to James Allen school in Dulwich for a foundation scholarship at 11.

“I was so naive, I didn't even realise it was a private school,” she says, laughing ruefully. “There I was with girls whose fathers were doctors and barristers, and we lived in a rented railway flat by Clapham Junction station which shook every time a train went by. When I had friends visit, they asked, ‘But how do you stand the noise?' We were so used to it, we said ‘what noise?'” That culture shock is perhaps what makes her especially acute on the feelings of her heroes and heroines, who also get plunged into a new world; but the Stravaganza series also plays on “the longest love affair of my life”, which is Italy.

Hoffman, who typically wears the kind of bright kingfisher colours that suit redheads, must have been a striking teenager; at any rate, when her parents first took her on holiday to the Italian seaside at 14, an Italian “god of a man of 19 who looked like Michelangelo's David” fell seriously in love with her and wanted to get married. Her Islington teenagers, who have the gift of “stravagating” or time-travelling to an Italy more Italian than the real thing, are equally passionate, encountering Jacobean plots, poisonings and struggles for power. (The delightful website, hosted by Bloomsbury, gives some idea of its complexity.)

In the first novel, City of Masks, the hero Lucien has cancer, and actually dies in our world but continues to live in Talia, something that worried me deeply at the time. Hoffman defends this choice (“It was always what Lucien's story was going to be - he didn't choose it and nor did I,”) and points to moving letters that she has received from mortally sick children or their friends. “If it comforts them, have I done a bad thing?” she demands.

Like many born writers, she encountered those connected to writing in the oddest ways - including having her appendix taken out as a child by Enid Blyton's husband - but it was not until she got to read English at Cambridge that she found her niche: acting, meeting future literary luminaries such as Clive James and going out with the future philosopher Roger Scruton (whom she describes as “gorgeous, like Jean-Paul Belmondo”). But when she came down, all she knew was that she didn't want to teach.

Eventually, she began to get the kind of jobs that sit well with writing fiction - becoming a companion for an elderly lady in Belsize Park, tutoring the young Artemis Cooper, and doing journalism for the TES. She wrote her first fantasy novel, White Magic, which was published with the help of Richard Adams.

“I went to interview him after Watership Down, and he offered to read it unsolicited, then sent it to his publisher, who offered me a contract. I thought, in my naivety, I'd arrived.” It received two reviews, and sank without trace, but she was “page struck”, as she puts it. Not until the success of Amazing Grace at the age of 46 was she able to write full-time; the concept of “stravagating” or being in two places at the same time comes, she says dryly, from being a working mother. Now, having moved from Crouch End to a barn in Oxfordshire because of her husband's job, she is making a living from her books, though the big breakthrough was the first Stravaganza novel in 2001. It came about because of a visit she made with her family to, of course, Italy.

“We went on a horrendously expensive gondola ride in Venice. It should have been perfect, but our gondolier, instead of being an Adonis, was a paunchy bald man. So I began to think ‘in what version of Venice would all the gondoliers be under 25 and handsome?' And of course, it would be in a city ruled by a woman - Bellezza. That was the original knot of ideas that led to Talia, though it took five more years to develop the Duchessa.”

A strong, fiercely independent woman, the Duchessa is, Hoffman says, “basically me but tall, beautiful, rich and powerful - a wonderful alter- ego”. Her family's struggle against the Medici-like di Chimici family has deepened as Hoffman has learnt to make her villains as well as her heroes more human.

Dramatic, enjoyable and as addictively readable to this generation as The Viper of Milan once was to their grandparents, they come from an author exploring the world of her rich imagination.

“I'm so bored by the battle between good and evil. That battle happens in here,” she says, thumping her own chest. “I love everything about books, it's in my blood, it's my metier. I'll never retire.”

Stravaganza: City of Secrets by Mary Hoffman
Bloomsbury, £6.99; 400pp Buy the book here

Times Recommends

• The 100 Best Books of the Decade
• The Times picks the 50 best paperbacks of 2009
• The conversation: James Ellroy