The Spider is one of the most violent hero pulps, with Richard Wentworth, aka the Spider, offing criminals left and right, while the criminals kill police and civilians wholesale. This is not a series for the queasy of stomach.
The title is slightly misleading--it's more like a secret city *block* of crime, but still very impressive. A series of bank robberies have occurred, each perfectly executed, and each involving the completely unnecessary death of an innocent bystander. And it's not just one gang doing this, with different robbery crews for each job. The Spider truth serums a man to death and learns of the Secret City of Crime. He then attempts to infiltrate this location.
From there on in it's pretty much nonstop action, and this would make a great movie. The Spider and his assistants are in constant danger, and there's an exciting sequence in the subway as the Spider attempts to prevent a horrific crash.
There are some great passages in this book; the description of the NYC subway at rush hour, the brief heroism of a little man in a derby hat, and the grotesque imagery of the villain's well-deserved death.