It is seven years since I first interviewed the writer Ian Rankin in a Bruntsfield cafe. During that time his professional life has been on a relentless skywards trajectory. Then it was charmed, now it is gilded. His reputation has grown from impressive to internationally renowned, and his wealth, irrespective of the credit crunch, has mutated from prosperous to fabulous.

In John Rebus, Rankin has created one of the enduring characters of British fiction — a detective to rival Sherlock Holmes. His novels account for 10% of British crime fiction sales and he has, almost single-handedly, invented a new genre: the Scottish crime novel. It has been seven years of gongs, awards and acclamation, punctuated by occasional invitations to Buckingham Palace, Downing Street and Vanity Fair parties.

I’ve seen him often enough in the interim to know that, physically, nothing has changed. The bloke-ish, studenty quality which has afforded him a degree of anonymity has been rendered almost freakish by its constancy. Two years off his half century, his hair contains no grey. It is as thick and geekish as it has always been. It isn’t so much that he has aged well as that he hasn't aged at all. It’s unnerving.

“I was asked recently when I started dyeing my hair,” he says over yet another cappuccino in yet another Bruntsfield café. “I don’t think they believed me when I said I didn’t. There comes a point when it’s not seemly not to have grey hair.”

He is just back from a tour of the US to promote his final Inspector Rebus book, Exit Music, and this weekend is a guest director at the Cheltenham Literary Festival. Intermittently, he stops talking to rub his eyes. It’s clear that he is dog-tired.

It is, however, inconceivable that this unremitting good fortune hasn’t rubbed off on him psychologically, not least because his highly developed fear of failure must have been vaporised by the scale of his success. If anything should have blown away his air of Presbyterian doom, it is the success of his first non-Rebus novel, Doors Open. A novella originally written as a serialisation for the New York Times, it has just been revised, extended and published to wide acclaim. The Guardian called it “brilliant” and the Financial Times deemed it “excellent new tartan noir”.

“I’m surprisingly pleased. I was waiting for the critics to put the boot in,” he says.

The novel’s position at number one on the bestsellers list, which would cause most authors to break open the Bollinger, elicits in Rankin only a sense of relief.

“It continues to show my publisher that there is potentially life after Rebus,” he says. “They are the ones who are worried. They've got used to a Rebus book a year and now we’ve taken the safety net away.”

But Rankin’s international following is huge. Doors Open was never going to flop. “My publishers have yet to be convinced that that’s down to me and not Rebus,” he says. Rankin is too canny and compassionate to kill off Rebus. “Apart from anything else it would have been an act of insane cruelty after all he’s been through.” he says. Yet Rebus still haunts his creator. Rankin has always had an ambivalent relationship with the character who has brought him fame and fortune.

He gets disturbed by the occasional letter from a reader asking him to investigate the death of a loved one in suspicious circumstances. “These are people who can’t distinguish between reality and fiction.” It’s a condition that occasionally afflicts the writer.

Rankin has an obsessive streak which borders on the compulsive, a need to control and be in control which explains why, despite an enormous correspondence, he does not employ an assistant. In the kitchen of his airy home in Merchiston, where his near neighbours are JK Rowling and Alexander McCall Smith, everything has to be in its place. When his wife, Miranda, does the shopping, Rankin checks off the list when she returns. He hates mess, a problem given that he has two children, Jack, 16, and Kit, 14. Kit has Angelman Syndrome, a rare genetic disorder characterised by severe learning difficulties and jerky movements.

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