You can already order the book here and the official summary goes as follows:
After a shootout claims the life of his partner in a condemned tenement building on Pearl Street, Detective John Tallow unwittingly stumbles across an apartment stacked high with guns. When examined, each weapon leads to a different, previously unsolved murder. Someone has been killing people for twenty years or more and storing the weapons together for some inexplicable purpose.
Confronted with the sudden emergence of hundreds of unsolved homicides, Tallow soon discovers that he's walked into a veritable deal with the devil. An unholy bargain that has made possible the rise of some of Manhattan's most prominent captains of industry. A hunter who performs his deadly acts as a sacrifice to the old gods of Manhattan, who may, quite simply, be the most prolific murderer in New York City's history.