 In 1934 Landau published his memoirs as a World War I spy master.In the book, "All's Fair", he revealed the existence of Karl Krüger, a former officer in the German Imperial Navy, who was one of MI6's most important World War I spies.Although Landau did not reveal Krüger's name, as Krüger was still active, MI6 considered Landau persona non grata.