If there has been a single constant thread running through all 12 of Alexander McCall Smith's No 1 Ladies Detective Agency novels, it has been the textbook guiding Botswana's one and only female private detective, Mma Precious Ramotswe.

The Principles of Private Detection by American sleuth Clovis Andersen has helped the "traditionally built" Precious solve many difficult and devious cases. She and her assistant, Mma Grace Makutsi, proud graduate of Botswana Secretarial College (97 per cent pass rate), naturally assume that thousands of private detectives worldwide constantly refer to the handbook, relegating him, in their eyes, to a genius almost on a par with someone like, say, Shakespeare.

Mma Ramotswe holds Clovis in the highest esteem - but she has no idea who he is, or indeed whether he is alive or "late", as they respectfully refer to the deceased in Botswana. But in the latest No 1 Ladies Detective Agency outing, The Limpopo Academy of Private Detection, she and Mma Makutsi receive a huge surprise in the form of a visitor - a tall, quiet, middle-aged American who hands over his business card which reads: Clovis Andersen.

The Clovis groupies are astounded and overwhelmed, especially when he murmurs that he had no idea his book was read in Africa ... or anywhere. "But we are always reading it," shouted Mma Makutsi. "Mma Ramotswe was the first, and then I read it, and then she read it again. It is always in use. Every day."

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Sadly it transpires that Clovis, a grieving widower on holiday from the United States, regards himself as a washout whose career, much like his book, never amounted to much. During the course of Limpopo Academy, however, he and Mma Ramotswe resolve a nasty situation using some of his textbook principles and, in the process, she boosts the self-esteem of her modest hero.

"I was delighted to meet Clovis Andersen," says McCall Smith, 63, on the phone from his home in Edinburgh where, he says, he is sitting with his Tonkinese cat Augustus Basil stretched out on his lap "looking very pleased with himself".

"We have heard so much about him and he is such a nice man. He was a failure really, poor chap, and she built him up and did so much for him. Mma Ramotswe was so nice to him when he said no one had ever bought his book and he wasn't a great authority at all."

McCall Smith says he decided to bring Clovis to Botswana because countless readers had asked about him over the years. "I have had people writing to me asking where they can get a copy of The Principles of Private Detection. I would say, the book doesn't exist."

Well, he could have told them the book had a limited print run, as Clovis points out to Mma Ramotswe. But that would have been a lie, not something the meticulous McCall Smith, who once served on Britain's human ethics commission, could possibly condone.

McCall Smith throws some seemingly insurmountable crises at Mma Ramotswe in the new book, which strike at her friends Mma Makutsi and her new husband Phuti Radiphuti, naive young Fanwell, her own husband's apprentice mechanic, and, most seriously, he delivers a crushing blow to her friend Mma Potokwani, the imperious matron of the children's orphanage farm.

Mma Potokwani's nemesis - leading to her sacking - comes in the form of "well-known businessman" Mr Ditso Ditso, who buys his way on to the orphan farm board and "treats it as a business", making cutbacks and spouting vile management-speak: "There comes a time when things must move on ... What's in a word? Resigned, dismissed, retired; jumped, pushed, shoved out? All the same at the end of the day."

When I point out that Ditso Ditso is a superb name for a rotten person, McCall Smith lets out a peal of one of his surprisingly high-pitched, charming giggles. "Ha ha ha - it is rather good, isn't it? Mr Ditso daring to get rid of Mma Potokwani, that's the worst. I was slightly surprised she didn't fight back - I think she was just a little tired and she's been outflanked.

"When he says things like, 'there comes a time when things must move on', the question to ask is: why? Why do we need to move on? I think most people actually don't want to move on, they are quite happy where they are. There's this extraordinary thing whereby people feel they have to change things when they are new in a job, whereas things might be going very nicely.

"Universities here have a big dose of that," adds the man who, until his writing took off about 10 years ago, had a successful career as a legal academic and professor of medical law at the University of Edinburgh.

"A new managerial group has taken over and they stop the academics trying to run them. They've got these managers and they change everyone's title so deans have been abolished in some places and I heard there was one university where students are described as units. UNITS!" he bellows. "It is absolutely appalling. So we need Mma Ramotswe to sort them out as well."

The great thing about writing fiction is that the author can get his own back on real-life people who have irked him, even though you know, in McCall Smith's case, the bash will be delivered with the utmost politeness. Enter Clarkson Putumelo, the man who will be building the new home of Mma Makutsi and Radiphuti, a man so rude at their first meeting on-site that he does not appear to notice that the notoriously prickly Grace is even there. A big mistake: "Mma Makutsi seethed ... [she] glowered with resentment," later leading to a discourse with her boss on the rudeness of some men and how to deal with them. Even worse is to come as the building proceeds.

"Builders were rather on my mind when I was writing that," McCall Smith admits. "I had terrible trouble with a builder over in the west of Scotland," where he and his wife Elizabeth have a house on a sea loch where they keep a 7m cutter.

But whenever does he find time to sail? At last count, McCall Smith's books - including the series of Isabel Dalhousie, 44 Scotland Street and Corduroy Mansions as well as the Lady Detective books, about 30 in total - have sold more than 40 million copies in nearly 50 languages. He spends a great deal of time each year on book tours and says he finds some of the travel "quite burdensome, but most of it is really fine".

"I am doing three really big American tours this year which is really a lot because it is plane after plane after plane. But last year when I did Australia, we did the Ghan Train down from Darwin to Adelaide, then we did a big trip through the Outback with friends from Melbourne. It was absolutely fascinating, the emptiness was absolutely amazing. I would like more time to do things like that but on the other hand I very much enjoy events when I can talk to readers of the books."

McCall Smith, who has two cousins in Nelson, has strong New Zealand connections - his Scottish paternal grandfather, a doctor, emigrated to the Hokianga in 1913 in disgrace (he had left his wife and run off with one of his patients), where he set up Rawene Hospital. Quite spontaneously, McCall Smith suddenly promises he will come here next year, following two earlier book tours of New Zealand.

"Let me say, I will definitely come. I will do it. Isobel Dalhousie drinks New Zealand white wine and somebody in the Scotland Street book I am writing has just had a glass of New Zealand white wine," he giggles.

There is no drinking wine in the offices of the No 1 Lady Detective Agency, just gallons and gallons of red bush tea. At one stage, as the two lady detectives drive into the Kalahari Desert, Mma Makutsi calculates that Mma Ramotswe drinks 100 cups of tea a week, or 5000 a year. Nothing to be ashamed of. "That bush tea is full of good things. It will be making me very strong," she tells her assistant as she swings her lopsided little white van on to the dirt track.

Mma Ramotswe drives what you might call an "honest" vehicle, but her husband, the mechanic JLB Matekoni, knows a "dishonest" car when he sees one. One like the car driven by Ditso Ditso, "covered in chrome and bits and pieces".

"He he he," laughs McCall Smith. "There are some cars where you think, 'that's a dishonest car driven by a dishonest person'. I think that's a theory you'd have to be a little bit careful with but it's quite an attractive one, I think, quite useful."

Useful enough, surely, to be added to a revised new edition of Clovis Andersen's The Principles of Private Detection.

* The Limpopo Academy of Private Detection (Little, Brown $34.99) is out now.