Think of a Number (Dave Gurney Series‪)‬

When Dave Gurney, retired NYPD homicide detective, is contacted by an old college acquaintance about some startling letters he's been receiving, it is a sinister puzzle. Then 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. This seems only to be the beginning. And the killer alone knows where it will end.