John Lutz: The Night Watcher (USA 2002)
From the Publisher:
As Stack and Lopez investigate the cruel deaths, they have no idea that they are being watched from the shadows -- observed by a cunning murderer picking up all the clues necessary to stay one step ahead of the police while perfecting a deadly craft. And when a pattern slowly emerges, the detectives realize that the killings aren't the random acts of a maniac, but the personal campaign of someone bent on retribution... someone who's been watching closely and knows their case too well.
John Lutz: The Night Watcher. Pinnacle Books, ISBN: 0786015152 (November, 2002), 397 p., $6.99.