She published her first book in her 40s and became the biggest selling author of the past decade in any genre – The Gruffalo alone has sold 13m copies. How did this former busker make it so big?

The room where the children’s author Julia Donaldson writes – the heart of her vast picture book empire – is down a winding staircase, in the cellar of her grand white house in Steyning, West Sussex. Her desk looks out on the street at knee height. “I’m thinking of writing a book about legs,” Donaldson said, as she showed me around the house this summer. Children from the nearby school often wave in at her as they pass. Donaldson is well known in Steyning, due to her frequent signings at the local bookshop. She and her husband, Malcolm, a retired paediatrician, recently bought the local post office to save it from closure. But elsewhere she can walk the pavement without being recognised. “I got a letter the other day for Jacqueline Wilson,” Donaldson told me. “It said: ‘You’re my favourite author!’”

If you are not someone who spends much time with young children, you may only be dimly aware of Donaldson’s work – although you will probably be familiar with her most famous creation, the Gruffalo. If you have children, however, you will know her as a cultural juggernaut whose influence among children is perhaps only surpassed by the works of Disney and CBeebies. Donaldson, who is 72, has written more than 210 books: chiefly picture books, but also poetry, plays, a 60-part phonics reading scheme and a novel for preteens. Many of them – such as Room on the Broom, The Snail and the Whale, and What the Ladybird Heard – are already considered classics. Several have been adapted into stage shows and animated BBC films (the latest, Zog & the Flying Doctors, will air on Christmas Day) and spawned an ever-expanding Donaldson universe of toys, clothing and merchandise.