True histories of western outlaws Jesse James and Butch Cassidy are paired in the classic Desperate Men. James D. Horan, the first researcher to be granted access to the long-sealed files of the Pinkerton National Detective Agency, was able to show in graphic and unsentimental detail the bloody desperation of the James-Younger gang and the Wild Bunch.   Horan reveals the insecure, bitter Jesse James behind the bandit's mask. His death ended a sixteen-year reign of terror in the Middle Border, but farther to the west Butch Cassidy, the Sundance Kid, and their cohorts soon loomed on the outlaw trail. Their criminal careers and intimate lives are tracked in this revised, enlarged edition of Desperate Men.