 Cooper's work was much admired by FBI Director J. Edgar Hoover, who once said he is "the best informed man on crime in the U. S." — even allowing Cooper access to FBI case files.Cooper is widely believed to have ghostwritten the book "Persons in Hiding" (1938) as well as a number of magazine articles for Hoover.A 1936 newspaper article in the "Chicago Daily Tribune" identified Cooper as one of Hoover's few close personal friends, along with Clyde Tolson and a man named Frank Baughman.