Think of a number

It begins with a letter. The letter contains a request - think of a number, any number - and a sealed envelope. When Dave Gurney, retired NYPD homicide detective, is contacted by an old college acquaintance about some startling letters he's been receiving, it is at first little more than a diverting but sinister puzzle. Until the acquaintance is brutally killed. Suddenly Gurney finds himself in the middle of a murder investigation that makes no sense.