In the Woods by Tana French

Detectives Rob Ryan and Cassie Maddox are called to an archaeological dig outside Dublin because a young girl's body has been found. The girl, Katy Devlin, had been reported missing the previous day. Going to the site is particularly traumatic for Rob because he grew up in the area and as a child he was found dazed and bloodied with no memory of the previous hours near the current murder site. His two best friends, Peter and Jamie, weren't so lucky. They vanished without a trace. When a hair clip seeming to belong to Jamie is found near Katy's body, Rob and Cassie wonder if the cases are connected. Rob treads a fine line working on the Devlin case, since only Cassie knows of his past. Will Rob and Cassie be able to find Katy's killer and determine what happened to Peter and Jamie? In the Woods is the first book in the Dublin Murder Squad series and is more a traditional police procedural than the third book in the series, Faithful Place.