The Miracle at Speedy Motors, by Alexander McCall Smith

"Fat woman beware! You think that you are Number 1, but you are Number Nothing!" The recipient of this anonymous letter is Precious Ramotswe, the "traditionally built" detective of Alexander McCall Smith's bestselling No 1 Ladies' Detective Agency series. In this ninth volume, Mma Ramotswe has to tackle a mystery of her own: who is sending these threatening letters?

It is 10 years since McCall Smith began the series, in which Mma Ramotswe solves mysteries through common sense and kindness. None of them could be described as standard detective fiction: there are no bodies or post mortems and the closest thing to a car chase is a trolley chase through a supermarket. All are rooted in Botswana – in the beauty of its "empty, blue, limitless" sky, its parched longing for rain, its traditions of courtesy and respect.

Speedy Motors is the garage of Mma Ramotswe's husband, Mr JLB Matekoni, where the detective agency has its office. The "miracle" is more complex.

At one level, it refers to Mr Matekoni's hope of curing the paralysis of their adoptive daughter, Motholeli. But at a more profound level, it refers to the "invisible links that connected people, that made for belonging". These links are revealed in an unexpected quarter – through the apprentice Charlie, "who was normally all jokes and showing-off" and who once called Grace Makutsi, Mma Ramotswe's assistant, a warthog. But seeing Mma Makutsi in distress, he suddenly "reached out and touched her lightly on the shoulder". It is deeply moving: a small miracle which shifts the "sandbank of animosity" between them.

Mma Ramotswe, as always, is at the centre of the narrative, providing the author's moral view. But the real star is Mma Makutsi. Up to now, she has been little more than a foil to her employer. But here her all-too-human failures – a slight untruth here, a too-hasty judgement there – compel our recognition and sympathy.

Although from a humble background, she obtained a stellar 97 per cent in her examinations at the Botswana Secretarial College. Despite her blotchy skin and large glasses, which made her the butt of the glamorous set, she is engaged to the owner of the Double Comfort Furniture Shop. She had "started life with nothing, or next to nothing", observes her creator with affection and pride, "and if she now had something, then that was entirely due to hard work".

So it was with Botswana: one of the poorest countries of the world at independence in 1966, but which – through hard work and prudence – went on to become the world's fastest-growing economy and a stable, middle-income nation. McCall Smith is clearly concerned to showcase Botswana's great achievements and the legacy of the founding President, Seretse Khama: "a man who believed in his country and had stood up for what it represented, which was decency above all else". If anybody in Botswana tried to rob a bank, comments Mma Ramotswe, "you'd probably know exactly who they were. You could simply threaten to tell their mothers."

The Miracle at Speedy Motors is written with grace and charm, just like the earlier books. But it is also strengthened by a new gravity: the message that Western nations – which tend to regard Africa as a hopeless mess – have in fact much to learn from Botswana and from its miracle of "belonging". The "holding of hands, human hand in human hand", urges McCall Smith, "could help, could make the world seem less broken".

Susan Williams's 'Colour Bar: the triumph of Seretse Khama and his nation' is published by Penguin