The Spanish Prisoner — Frank Gruber

Another first-class thriller from the author of BROTHERS OF SILENCE. He knew it was one of the oldest con games in the world - the Spanish Prisoner racket. An ex-FBI man can't resist a mystery, especially if he's lonely and miserable in Spain, haunted by the stricken face of a twelve-year-old girl. Yet someone had sent him a letter with a 1520 gold coin attached. He had to know who - and why - no matter where the chase might take him.