A Canadian novelist Frank Lucius Packard (1877-1942) wrote about 30 thrillers including ones with the character Jimmie Dale, aka Gray Seal and Smarlinghue, a safecracking Robin Hood who uses his criminal talents to right wrongs, save lives and reputations, and expose wrongdoers to the light of day. Jimmie Dale and Blue Envelope Murder is 4 and the last book in the Gray Seal series. In it, Jimmie Dale tries to protect his friend who received a threat in the form of a mysterious blue envelope, but when the friend is found dead, Jimmie is accused of the murder. To clear himself, Jimmie must resolve the envelopes mystery and find out who stands behind the murder, and he must do it while avoiding the police and his old enemies from the underworld.