Riley by Christopher Harlan

On Staten Island, New York, a series of horrific murders have struck terror into a local, tight knit community. Four victims, all related, and no one willing to say the two words that would shed light on the case —serial killer. When the local police make no headway on the case, Riley, a Bronx homicide detective, is called in to help.
Riley is an experienced, hardened, and brutally honest NYPD detective. Now free from the responsibilities of helping his friends Quinn and Noah track down the Network, Riley is called to duty once more to help seek justice for the victims of the Staten Island murders. At a crossroads in his life, Riley is secure in his work as a detective but struggling with his personal life. A string of short relationships has left him unfulfilled and wanting more, and he has to find a way to balance the gruesome case before him with his own quest for happiness.