In 1984, urban photographers Martha Cooper and Henry Chalfant came together to publish an era-defining book about the early graffiti movement.They had been documenting the work of graffiti arts on the subways system of New York for many years.The colourful book was called Subway Art and it quickly became known as the graffiti bible.Forty years on from its release, Martha and Henry explore its enduring legacy with Matt Pintus.(Photo: Marta Cooper and Henry Chalfant pictured at the 25th anniversary event for Subway Art. Credit: Getty Images)