Martha Cooper is an icon of street art. She is also a small, grey-haired woman who at the age of 75 still sneaks out with masked graffiti artists to photograph and perpetuate their works. In 1970s New York, she was one of the first people to document the graffiti of the city's trains, which she published in the book 'Subway Art'. But the publication's commercial failure forced her to turn her back on graffiti. 20 years later, Martha had become a legend in the street art scene and her book, which inspired an entire generation of young people, now ranks among the best-selling – and most stolen! – art books in history.