Missing person / Sarah Lotz.

Enhanced descriptions from Syndetics:

From acclaimed thriller writer Sarah Lotz, hailed by Stephen King as "vastly entertaining," a new novel about a group of amateur detectives infiltrated by the sadistic killer whose crimes they're investigating.
Reclusive bookseller Shaun Ryan has always believed that his uncle Teddy died in a car accident twenty years ago. Then he learns the truth: Teddy fled his home in Catholic, deeply conservative County Wicklow, Ireland, for New York and hasn't been heard from since. None of Shaun's relatives will reveal why they lied about his uncle's death or why they want Shaun to leave the whole affair alone.

But Shaun has a burning need to find out the truth. His search is unsuccessful until he's contacted by Chris Guzman, a woman who runs a website dedicated to matching missing-persons cases with unidentified bodies. Chris and her team of cold-case obsessives suspect that Shaun is looking for the "Boy in the Dress," one victim in a series of gay men murdered by the same killer.

But who are these internet fanatics really, and how do they know so much about a case that has stumped police for decades? Soon armchair sleuths and professional investigators are on a collision course with a sadistic serial killer who's gotten away with his crimes for far too long - and now they're in his sights.