IF HE HAS A RAGING EGO, extreme vanity or hopeless insecurity, or, indeed, any of the other traditional writerly frailties, Alexander McCall Smith keeps them well hidden. He is charming, avuncular, a global publishing phenomenon who looks like a Rotary Club chairman. His No 1 Ladies' Detective Agency series has sold more than 15 million copies in English and has been translated into 42 other languages. His other series - about the philosopher sleuth Isabel Dalhousie and the goings-on at 44 Scotland Street - are also hugely popular.

The ninth Ladies novel, The Miracle at Speedy Motors, is a gentle tale of family secrets and discoveries, with the anchoring presence of the “traditionally built” Precious “Mma” Ramotswe whose common sense and lightly deployed powers of detection guarantee a satisfying conclusion.

When does the 59-year-old writer find time to work? He lives in Edinburgh but is constantly on tour, and has recently returned from Spain, Germany and a whistle-stop sortie to Devon and Cornwall. “I write while I'm on the road,” he says. “I finished volume five of my latest 44 Scotland Street on the train from Exeter.”

McCall Smith notes gloomily the “soullessness” of airports; how he meets so many people “but at the end of the day you're alone”. He's a sucker for a hotel's house Caesar salad and worries mildly about the effects that all these have on his weight. His wife Elizabeth, a doctor who has just retired, joins him for some of the trips; his two daughters, Lucy and Emily, are medical students.

“I enjoy the fact that the books have found so many readers,” McCall Smith says. “That's very nice. I enjoy the sense of being in a conversation with readers. But you lose a certain amount of anonymity and control over your time. I would like to lead a more normal life. But it's difficult to say no, to detach yourself.” Such is their popularity that he feels he has “ceased to be in control of the characters. People are very proprietorial about them. It's a machine which carries you forward. It takes considerable determination to control it.”

McCall Smith tries to fence off months to himself, traditionally January and February, “but this year a coach and horses was driven through that because I went to a literary festival in Sri Lanka”. In the summer he will go to Botswana - he goes every year party because he loves it, partly to get fresh material for the novels - to open a small opera house, in a converted garage which will seat 60 and act as a showcase for “local singers who wouldn't otherwise have a chance to sing in public, with a chamber orchestra of ten to 12 players”.

He is passionate about the “openness of Botswana's skies, its striking physical beauty” and the “remarkably nice” people. There's so much “bad Africa” on the news, which, while vital to report, he feels doesn't tell the whole story.

It's an amazing piece of ventriloquism: white Scottish man inhabits fictional soul of black African woman, I say. Not really, he replies: “As a writer you try to empathise and imagine the experience of ‘the other'.” He doesn't think gender and race are “insurmountable obstacles”, though “if I was attempting to write a critical book, or intense social realism, I'd have to be extremely careful. I'm writing something like a fable. Of course, it distils an aspect of reality. If we went to Botswana I could introduce you to women like Precious.”

McCall Smith has some telling literary heroes: Auden, “who has such humanity and is so sensitive to meter. His literary executor wrote to me to tell me that he and Precious would have agreed on everything.” He likes Barbara Pym, too (“she celebrates small things and perhaps so do I”). He says he “almost goes into a trance” when writing: “All fiction comes from the subconscious mind.”

Are he and Precious alike? “Well,” he says dryly, “she says nothing I disagree with”, though Isabel Dalhousie shares his view of the world more closely. Could the detective agency ever close its doors? “I think knowing when to leave the stage is important. I am under contract to write 11 books. Speedy Motors is number nine. I may carry on with them. The beauty is having the very loose generic framework of a detective story means anyone can walk through the door.” Will Precious die if and when the series ends? “No,” McCall Smith says firmly, “she will just drift into a happy retirement.”

McCall Smith (he is known as Sandy, rather than Alexander) had a “pretty average” childhood, growing up in Southern Rhodesia, now Zimbabwe. “I was fairly bookish with an intense imaginative life.” He studied law, and taught it in Africa, and became Professor of Medical Law at Edinburgh University.

His writing began with children's books. “I was in my mid-40s, with a small coterie of readers, and I thought that was where I'd remain. The first Ladies was published very small-scale.” Success has meant “the freedom to write what I want to write”, and he wants now to flirt with literary fiction - following Dream Angus, a contemporary reworking of the ancient Celtic myth of the dream-giver god, in November he will publish La's Orchestra Saves the World, a novel based on stories he wrote for Radio 3 about a Suffolk woman and the amateur orchestra she founds. “I don't think there is a rigid distinction between the literary novel and the popular novel,” he says. “I have a very broad readership and don't feel ashamed of my books at all. I think they reassure people. People feel happier at the end of them. I don't think they read me for excitement. If they do, they're reading the wrong author. In an important sense the books are comfort reading. I get moving, humbling letters saying how they have helped readers at difficult times. A woman wrote: 'My husband and I read this while he was dying, it was the last book he read.'”

McCall Smith pulls out two letters he was slipped in Exeter the night before. One thanks him for the “beauty and simple profundity” of his books. “You learn how deeply involved people become in fictional characters and you have to take that seriously,” he says.

McCall Smith's popularity will only grow with the screen adaptation of the first Ladies novel, written by Richard Curtis and directed by Anthony Minghella (and starring the singer Jill Scott as Precious Ramotswe), to be shown on BBC One on Easter Sunday at 9pm - the first in what is expected to be a series. Watching some rushes with Minghella during filming in Botswana, McCall Smith cried. “Anthony put his hand on my shoulder and said, ‘It did that to me too'. Apparently lots of the crew have been finding it moving.”

McCall Smith's schedule and output are staggering (and envy-inspiring). “Next I am writing volume five of Isabel Dalhousie; I'll start volume ten of Ladies in July, finish that in October, then start the next Dalhousie.” He never gets exhausted, or suffers writers' block. “I think that's just another way of saying you're depressed,” he says. Has he ever suffered from depression? “No, touch wood. Um, I was mildly low, once in Italy when I was a visiting professor. Homesickness.”

He says he is healthy, though “of course” has an awareness of mortality - “who doesn't when you're above the age of 25? You're only immortal until you're 18. I don't dwell on it and I'd avoid it if I saw it coming my way. The major challenge is to enjoy, and help others enjoy, the all too brief moment we are here.”

He is “sympathetic” to people with religious beliefs, but alludes to Camus: be prepared to discover that the meaning of life is limited. He is fulfilled, mostly. “There must be some things I haven't done or haven't been able to do. I have a few regrets - that I cannot play the piano, for example. To have that fluidity pianists have ... And I would love to be able to paint.”

But it's never too late and, inspired by the bracing nautical fiction of Patrick O'Brian, he and Elizabeth have learnt to sail and conducted their maiden voyage on a 30ft boat off the Canadian coast. “It was quite an achievement but we didn't lose sight of land,” he says, laughing. “I had a captain's cap with an anchor crest, a book of international signalling codes and some signalling flags. One, which I kept close to hand, meant 'I'm not in control of my vessel'. It's a good flag to have in life generally.” But, given his success and exacting work ethic, one that he doesn't have much need for.

The Miracle at Speedy Motors by Alexander McCall Smith

Little, Brown, £14.99; 256pp

Five fascinating things about A.M.S

1. His academic publications include The Criminal Law of Botswana and The Forensic Aspects of Sleep, about the legal implications of such phenomena as crime during sleepwalking.

2. He plays contrabassoon in the Really Terrible Orchestra, which he founded with his wife in 1999. He has passed Grade IV, but still has trouble with C sharps.

3. When he visits Italy he brings home to Scotland enough parmesan to last six months.

4. He got the idea for Precious Ramotswe from watching a Botswanan woman chase a chicken, catch it and wring its neck. “I thought to myself: I would like to write about an enterprising woman like that.”

5. During the making of The No1 Ladies' Detective Agency film, he briefly played director: “Minghella allowed me to shout 'Action!' and 'Cut!' That was my movie career.” The star of the scene was a donkey.

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