Stephen Sackur talks to a writer whose work has made him the most dangerous of enemies. A decade ago Roberto Saviano wrote a best-selling book, Gomorrah, which exposed the power and brutality of the Neapolitan mafia. The crime bosses put a price on his head, and he has lived in a shadowy world of safe houses and bodyguards ever since. Now he's written a new book about the global cocaine trade - why has he sacrificed so much to expose organised crime?