'Despise it not': a Hull man spies on the Kaiser's Germany by Ian Sumner (Beverley, Highgate, 2002; ISBN 1902645340)
The life and espionage career of Max Schulz, an Englishman of German parentage, who spied on the Kaiser's navy in the years leading up to the First World War. He was captured in Hamburg in 1911 and imprisoned in Fuhlsbüttel prison until 1918, when he simply walked out of the jail at the end of the war. Based on his own account of his time in prison, and on German archive material.