 Whether you love it, hate it, think it's art or vandalism, you'll spot graffiti all over the world. And in recent years, street artists like Banksy have been able to make millions from their work. But that wasn't always the case. This month marks 40 years since the release of a book that put street art firmly on the map. A book that became known as the Bible of Graffiti. This is the story of Subway Art.