In the woods [sound recording] / Tana French.

Enhanced descriptions from Syndetics:

When he was twelve years old, Adam Ryan went playing in the woods one sunny day with his two best friends. He never saw them again. Their bodies were never found, and Adam himself was discovered with his back pressed against an oak tree and his shoes filled with blood. He had no memory of what had happened. Twenty years later Adam - now using his middle name of Rob - is a detective with the Dublin police force. His colleagues don't know about his past. He works as a team with Cassie Maddox, a smart, tough cookie; they are best friends as well as partners. When the body of a young girl is found at the site of an archaeological dig, Rob and Cassie get the case. And when they reach the crime scene, Rob realizes it is the exact site of his childhood trauma. They also find a hairclip that he recognizes as having belonged to his friend. Could there be a connection between that old, unsolved crime and this? Knowing that he would be thrown off the case if his past were revealed, Rob takes a fateful decision to keep quiet. Rob and Cassie are investigating the murder of Katy Devlin, but they both hope that they might also solve the twenty-year-old mystery of the woods.