Frank L. Packard - Adventures of Jimmie Dale

Frank Lucius Packard (February 2, 1877 – February 17, 1942) was a Canadian novelist born in Montreal, Quebec. He worked as a civil engineer on the Canadian Pacific Railway. He later wrote a series of mystery novels, the most famous of which featured a character called Jimmie Dale.

Jimmie Dale is a wealthy playboy by day, with a Harvard education and membership to New York City’s ultra-exclusive private club St. James. But at night he puts on a costume and becomes The Grey Seal, who enters businesses or homes and cracks safes, always leaving a diamond shaped, grey paper «seal» behind to mark his conquest, but never taking anything. He was just doing it for «the sheer deviltry of it» at first, but when a woman catches him, she blackmails him to war on certain crime organizations.

Jimmie Dale/The Grey Seal is often credited with greatly influencing and popularizing later pulp and comic book heroes. The foppish playboy-by-day-crimefighter-by-night routine had a precursor in The Scarlet Pimpernel, but it was Jimmie Dale that brought the idea into a contemporary setting and added the idea of a costume and mask for his secret identity, serving as a possible influence for characters like Zorro and The Shadow. He also established the concept of a hero's secret hideout or lair, The Sanctuary, a precersor of the Batcave or the Fortress of Solitude. (Summary by Wikipedia and Maire Rhode)
The sequel to this book, The Further Adventures of Jimmie Dale, can also be found on Librivox.