Over the past ten years Jacqueline Wilson has been the most borrowed author from British libraries. She's sold 30 million of her books just in the UK - and written nearly a hundred of them over the years - girls love them. They almost always focus on a young girl in a difficult family usually being brought up single-handedly by her mother, sometimes with an abusive stepfather. And often featuring drink or drugs. So why does she draw on such bleak territory?Jacqueline Wilson talks to Sarah Montague