It turns out success does have its price. When US author Janet Evanovich heads off on a publicity tour, she takes with her a large supply of the painkiller Advil. With up to 5000 fans turning up to each signing session, all of them buying multiple copies of her books, that adds up to a lot of inflammation in her signing hand. What Evanovich's fans go crazy for is the blend of comedy, romance and adventure they're guaranteed to find in every single one of her books - particularly the Stephanie Plum novels - the latest of which is Sizzling Sixteen (Headline, $38.99).

"I've always written with humour," Evanovich tells me over the phone from her home in Florida. "It's where I stand on the planet. I see things differently to other people."

Evanovich started out as a stay-at-home mum, writing romance novels in her spare time and collecting a box-load of rejection letters from publishers. She was at the point were she'd given up, burned the box and signed on to a temping agency when she finally had a manuscript accepted. Twelve romance novels later she ran out of steam with the genre.

"I loved it but in the end found I had a hard time getting out 300 pages without bits of mysteries and adventures finding their way in," she explains. "Also I was never particularly good at the hands-on sex scenes. So I moved into crime but took the best bits of romance with me - like the humour and the sexual tension - and squashed them into the mystery format."

Although she's written other series, the Stephanie Plum novels are what Evanovich is best known and loved for. Essentially girly romps, they feature Plum as a hapless New Jersey bond enforcement officer with a sleazy boss, an eccentric granny, two hot guys vying for her affection and a junk food-loving, sassy sidekick called Lula.

Sizzling Sixteen sees the boss, Vinnie, kidnapped and Plum and her pals trying to rescue him before he's eliminated - but the plot almost isn't as important as the fun Evanovich has along the way. "I make a promise to my readers with every book," she explains. "I deliver a page-turning, entertaining experience with positive characters, not too much onstage violence and no explicit sexual content."

The first Plum novel, One For The Money, changed Evanovich's life. Before the book even hit the shops, the film rights were sold for a million dollars and her maths professor husband gave up his job to support her while she wrote full time. That was back in the early 90s and as yet there's been no sign of a movie. "But they're starting to shoot it next month," Evanovich tells me excitedly. "And I'm thrilled because Katherine Heigl is going to star as Stephanie Plum."

Writing is very much the Evanovich family business. Her son Peter is her agent and her daughter, Alex, runs her website and the co-author programme which sees Janet collaborating with other writers so she can put out even more books to satisfy her fan base.

"We're like a herd," says the author. "We live close together and have lunch together every day so we can brainstorm ideas if I'm stuck. Working together is great. We actually like each other which is the amazing part. But there's also a staggering financial responsibility on me and it took a while to get used to that."

Evanovich works long and hard at her writing. "I'm very boring," she says. "I'm at my computer by 5.30am and find it easy for the first couple of hours and then it gets more difficult as the day wears on and my mind empties of ideas. The humour comes very naturally. Hands down the most difficult part is the romance. It's difficult to keep the tension and attraction going without finalising it."

She stops writing at 4pm which is when her personal trainer arrives. "I hate any exercise that doesn't involve walking through a shopping mall," Evanovich confesses. "But last year I gained 30 pounds and really blimped out after falling off the stage at a Toronto signing session and breaking my foot. So now I've hired a cook to measure out my food and keep me on 1200 calories a day and a physical therapist who makes me sweat."

She may be in her 60s, but Evanovich has no plans to slow down. This year she will release a graphic novel she has co-authored with her daughter, write a spin off series for a character called Diesel who has proved popular (the first book is called Wicked Appetite) and, of course, work on another Plum novel because the audience for them continues to grow.

"It all adds up to about three jobs," laughs Evanovich. "I can't decide if I have no life or too much life. But I guess I must like it. I was supposed to be taking three months off to relax, lose weight and get a grip on myself but after two weeks I was going nuts and kept sneaking back into my office. I have all these story ideas in my head and the world's best job."