Think of a Number (en Inglés) - John Verdon

It begins with a letter ...The letter contains a request - think of a number, any number - and a sealed envelope. Inside the envelope is that number. 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. The killer seems to have known his victim intimately. How else was he able to predict his victim's thoughts, even his actions? How did he know his darkest secrets? The killer is smart and he is playing with the police. Gurney needs to be smarter if he's going to catch him, but this seems only to be the beginning. And the killer alone knows where it will end.