A few weeks ago I got a historical novel, written for adults, called Little, based on the life of Madame Tussaud. I soon learned that my 12-year-old son had beaten me to this author’s work: He’d already read Heap House, the first novel in the outlandish, fantasy-based The Iremonger Trilogy, aimed at precocious kids. I was lucky enough to speak to the writer, Edward Carey, about how he kept it all straight, and how slight the differences between categories are.