When I was seven, or thereabouts, my father left my mother, and moved to a less genteel part of the city, where he lived in a tiny terrace opposite a potted-meat factory. No one had told me that another woman was involved in my parents' separation, but I think I'd guessed as much – and, sure enough, following a few token solo weekends, during which my father lavished us with Lego, salted peanuts and orange squash, I was introduced to the girl, two decades his junior and only just out of university, who was to become my stepmother. It goes without saying that this meant all sorts of things, emotionally. But it changed things culturally, too, even if I wouldn't have put it like that at the time. M brought with her a passion for guinea pigs, a large and somewhat terrifying collection of prog rock albums and a stack of slightly battered books, which, thanks to her age and background, were beguilingly different from those on my mother's shelves. In the coming months, determined to be my friend, she duly handed over the favourite titles of her childhood. I should try them; she promised that I wouldn't regret it if I did.
At first, I wondered about her taste. The Little Grey Men by 'BB', a Carnegie Medal winner from 1942 in which four Warwickshire gnomes embark on an epic journey through the English countryside, was a struggle and a joke, and I didn't finish it. Nor did I much care for The Starlight Barking by Dodie Smith, the sequel to One Hundred and One Dalmatians (the humans of the world have fallen asleep; Pongo and Missus investigate). Even as a child, anthropomorphism left me cold. But Emil and the Detectives by Erich Kästner … now she was talking! Racing through her old Puffin – like every edition since the novel was first published in 1929, it came with the delightful illustrations of Walter Trier – I felt a powerful sense of ownership, as if I'd ordered it up, chosen it like an ice-cream from a van. Like me, Emil found himself shuttling between A and B. Like me, he worried about his mother, who lived alone. Like me, he was keen to play detective (I was always informing my father that, in the light of another murder having occurred in the house, I would have to take his fingerprints; make of that what you will, dear Freudians). Hell, there was even a scene in which our boy hero ended up in the compositors' room of a Berlin newspaper "where there was a clatter like a thousand typewriters at work all at once" (heaven to me because I already dreamed of being a journalist). Emil and the Detectives was, in my view, the perfect book, and the fact that it was written by a (to me) exotic-sounding German and was then rather less well known among my peers than, say, Stig of the Dump or Tom's Midnight Garden only added to its allure. I enjoyed – to pinch from James Wood – a sense of clandestine fellowship with it. I decided to give M the benefit of the doubt.
The story goes like this: Emil Tischbein lives in Neustadt with his widowed mother, a hairdresser. Mrs Tischbein worries about making ends meet, an anxiety that she transmits, unwittingly or otherwise, to her devoted son. So when she sends him off to stay with his aunt and grandmother in Berlin with £7 in his pocket, he is determined not to lose it. Not content with keeping the money in an envelope in his suit jacket, he pins the notes to its lining, something that proves to be highly reassuring when he finds himself sharing his compartment on the slow train to the capital with a sinister-looking man in a bowler hat called Mr Grundeis. But then he falls asleep – his eyes simply will not stay open – and by the time he wakes up, all is lost: Mr Grundeis has disappeared, and so has Emil's precious cash.
Emil, though, is a preternaturally adult kind of a boy, and when he spots Mr Grundeis's hat in the crowd at the Berlin station at which the train has stopped, he resolves to follow him and get his money back – no matter that his grandmother and cousin Pony are patiently waiting for him. Happily, he isn't alone for long. While Grundeis is eating his lunch in a restaurant, Emil meets a boy called Gustav. Intrigued by Emil's mission, Gustav rounds up his gang – using an old-fashioned motor horn – who agree to form a crack team of detectives; at their head is the Professor, who wears horn-rimmed spectacles and has the wise manner befitting the son of a judge. Pretty soon, they have Grundeis surrounded, and soon after this – not to give too much away – he gets his comeuppance. Emil and his new friends, meanwhile, are now heroes, the stars of newspaper headlines.
Though I haven't seen it yet, it's no surprise to me that the National Theatre is staging a theatrical version of Emil for its Christmas show, adapted from the novel by Carl Miller. It's such an exciting, well-paced story, and its sense of movement – the boys form an intricate, highly organised web in which to catch their prey – doubtless lends itself perfectly to the large Olivier theatre. But reading it again, for the first time in 30 years, I see, too, what an amazingly shrewd and subtle writer Kästner was, and I can only urge you, now, to send your children – or any children you know – back to the book itself, irrespective of whether or not you're planning an outing to the play. Kästner remembers all too well the longing that children have to be adults, and he sets his characters thrillingly free. When they ask their parents if they might stay out all night, the better to bunk down close to Grandeis's hotel, they are not refused. And why would they be? The adults take them seriously; they trust their children's judgment, and assume that this faith will be repaid. "[My mother] lets me do more or less as I like," Emil tells the Professor at one point. "But then I mostly don't want to, if you see what I mean." More poignantly, Kästner understands the way children absorb not only their parents' worries, but their position in society. In his world, childhood isn't experienced by those who inhabit it as a class-free nirvana. Quite the opposite. Every boy knows his place in the pecking order. When the Professor tells Emil that no one in his house ever talks about money, Emil replies: "Then I expect you have plenty."
Its author, of course, knew whereof he spoke; there's a reason a journalist called Mr Kästner makes an appearance towards the end of the novel. Like Emil, he was an only child, and his mother, Ida, too, was a hairdresser, a job she did to supplement the income of his saddlemaker father. He and Ida were, moreoever, unusually close, and when he left home – he was drafted into the German army in 1917, and enrolled in university at Leipzig in 1919, after which he headed for Berlin to become a journalist and poet – he wrote to her daily. In Emil and the Detectives, Ida's presence is strongly and tenderly felt. Mrs Tischbein is reluctant to allow her son to treat her with the gift of a new hairdryer with the money he receives as a reward for helping to secure the arrest of Grundeis. But on her way to meet Emil in Berlin, she reads the newspaper story in which he appears no fewer than 11 times.
In the back of the Vintage Classics edition of the novel, sitting slightly uncomfortably between two quizzes, is a short biography of Kästner informing young readers that his books – he wrote for adults, too – were labelled anti-German by the Nazis, and that he was present as they were tossed into the fire by Goebbels in 1933. My stepmother's Puffin edition did not include this information (nor, come to that, did it explain that the prolific Kästner was a pacifist and, later on, an alcoholic). But I knew enough about the war, the defining event of my grandparents' lives, to know that a German writer publishing at a time when trams were still around – for this was how I dated the book – was bound to have been involved one way or another, and this, for me, gave the novel an extra edge of danger. It was easy to conjure Grundeis in my mind's eye; I pictured him looking like the baddie in a Sunday afternoon black and white film.
But this is the other important thing about Emil and the Detectives: Kästner set his novel in contemporary Berlin, a city of apartments and underground trains. At the time, this must have seemed a touch outrageous. Most children's books were set either in a fantasy land, far removed from the sound of steam trains and rush-hour crowds, or in nurseries, literally or metaphorically. Kästner eschewed such cosiness, and, in doing so, more or less single-handedly invented a new genre: the child detective story (though the teenage Hardy Boys and Nancy Drew emerged across the Atlantic at about the same time).
Its huge success – never out of print in English, it has so far been translated into 59 languages – paved the way for so many others, from Enid Blyton to Anthony Horowitz. In my own case, Blyton was the very next author I picked up; my mother had nearly all the Famous Five stories in hardback, and I read them one after the other in a huge binge. This mania for detection turned out to be a wonderful thing. Blyton, in turn, led me to Conan Doyle's Sherlock Holmes stories – I must have read "The Adventure of the Speckled Band" a dozen times before I was 12 – and thus, a new horizon opened up before me: the realm of adult books. It was official. I was a reader.
Last week, as I reread Emil, all that I have described to you came back to me in a "whoosh": not just the book's pleasures and consolations, but the way its hero somehow helped to form me. I had half-forgotten this. I'm not one of those people who thinks "great" children's literature and "great" literature are the same thing, and when the University of Kent's creative writing programme recently apologised for having dared to distinguish between the two, I was amazed. But this isn't to say that I think children's books aren't deeply important to their readers. For me, they were more vital than I can possibly say. "Password Emil!" The phrase worked for Kästner's boy detectives and, in the end, it worked for me.
• Emil and the Detectives is at the Olivier theatre, London SE1, until 18 March.
