The French economist's new book, Capital in the 21st Century , is flying off the shelves and topping bestseller lists - a sensation almost unheard of for a book thick with data and complex analysis. It's spurring heated debates around the world about the nature of wealth and inequality. The Guardian's Tom Clark - author of his own new book on the topic - meets the so-called 'rock-star economist' during a whistle-stop tour of London