IN THE mid-1980s John Grisham, then a small-town lawyer and disillusioned member of the Mississippi state legislature, would fill the time between meetings and court hearings writing a novel about an ambitious young lawyer embroiled in a life-or-death fight for truth and justice. "It took me three years, and most of the time I thought I would never finish it. Eventually 5,000 hardback copies were printed and I was thrilled. But they did not sell out, it did not get a second edition, it was not published in paperback or picked up for foreign rights. Then I wrote The Firm ."

The Firm, Grisham's 1991 story of another young lawyer in a jam, was on the New York Times bestseller lists for 44 weeks, sold more than seven million copies and was made into a feature film starring Tom Cruise. "My first publishing experience was entirely normal and my second entirely abnormal," he says. "I responded much better to the second experience than I did to the first."

And, for Grisham at least, the "abnormality" of The Firm's commercial success soon became the norm. His subsequent series of legal thrillers has gone on to sell close to 300 million copies and been translated into 40 languages. Nine of his novels have been turned into films starring A-list actors such as Julia Roberts, Gene Hackman, Sandra Bullock, Susan Sarandon and Dustin Hoffman. They have been directed by Sydney Pollack, Francis Ford Coppola, Joel Schumacher and Alan J Pakula. Grisham regularly features on literary rich lists with an estimated fortune of US$600 million and an annual income in the tens of millions. For most of the 1990s Grisham Day, when his new book went on sale, was a fixed point of the publishing year to be avoided by other publishers and celebrated by bookshops.

"My name became a brand and I'd love to say it was the plan from the start," he says. "But the only plan was to keep writing books. And I've stuck to that ever since." His 24th, The Litigators, was published in the UK in October. Lighter in tone than much of his work, it features a pair of morally dubious Chicago street lawyers, Finley & Figg, who find themselves teamed with a young, burned-out corporate lawyer, David Zinc, in an unequal battle against big pharma. Grisham first developed the idea as a sitcom script.

"The humour was there from the start. When you work at street level you never know who's going to walk through your door. Life is full of fun stuff, sad stuff and crazy stuff. I survived as a street lawyer for 10 years and bumped into guys like these. I got to know them quite well."

But a few things have changed since Grisham began to practise law 30 years ago. "Back then the term 'ambulance chaser' was very derogatory. You might sneak around trying to get cases quietly, but you didn't want people to know that. This was all before TV and billboard advertising which, in America, is now out of control. It used to be that your reputation brought you clients." He says the nature of the work he took on meant that sometimes he got paid and sometimes he didn't. "I had a lot of trouble saying no and therefore I never made that much money. But you always had that chance, as my character Oscar says, of a good car wreck. These days someone who's had a car wreck is lying in hospital watching TV, they see an ad and can call a lawyer. But that guy can't try your case. He's a lousy trial lawyer and afraid to go to court. It's just a volume thing. To make as many settlements as they can, which is not always in the interests of the person who has been injured."

Grisham has occasionally moved away from the legal world in his novels, and has also made sorties into non-fiction in a book about a miscarriage of justice short stories and, recently, children's fiction. But all his work has a concern for social issues and often deals directly with the legal and moral questions around such matters as the death penalty, homelessness, health insurance and prison conditions. Like the best crime fiction, his books often focus on where society is broken, and while he prefers not to call himself a liberal "I am a moderate Democrat" he remains politically engaged. As a strong critic of the Iraq war he was delighted to see Bush leave office, but he was, and remains, wary of Obama.

Grisham was born in 1955, the second of five siblings, and was brought up on an Arkansas cotton farm. "We kids didn't really realise just how bad things were," he says, "but the first 10 years of my life were lean times for cotton farming and there was not much money around." The family then began to move round the south as his father took up various different jobs, and within a few years things had improved. "My father worked seven days a week, sometimes 12 hours a day, for a construction company. He would take all the overtime the company would give him and we started to do well as a family. That sense of hard work bringing rewards was very much instilled into all of us."

Grisham's literary hero was and still is Mark Twain. "I wanted to be Tom Sawyer. I loved that romanticised view of a kid's life. It wasn't until a lot later that I realised there was more going on with Tom and Huck than just an adventure." Steinbeck was also important to him, as was Dickens, and he has been gratified by several critics praising the "Dickensian" feel of The Litigators. "And, of course, I read Faulkner. If you grow up in Mississippi you have to. You are force-fed him at high school, but I never got on with it that well. I do appreciate his genius. No one can do the sights and sounds of that part of the world quite like him.

His description of the smell of walking into a country store is perfect, but I also thought he was intentionally vague and obtuse. Maybe I just don't think reading a book should actually be hard work."

Observer