When Adolf Hitler was in power, the Nazis systematically plundered artwork and all manner of cultural property from the many territories they occupied. At the heart of that operation was Hildebrand Gurlitt, perhaps one of the greatest art thieves in the world. He successfully amassed over 1,400 pieces of art, including pieces by Chagall, Picasso and Matisse. Part of that collection, valued at 1.4bn, was only discovered 80 years later, in the apartment of his son Cornelius Gurlitt. We speak to author Susan Ronald about this notorious cultural heist.