Nobody's Fool (Detective Kierce 02) by Harlan Coben

A year after the devastating events that took place in Fool Me Once, a secret from former Detective Sami Kierce's college days comes back to haunt him. Present day is hard enough for Kierce, but his past isn’t through with him yet…
Sami Kierce, a young college grad backpacking in Spain with friends, wakes up one morning, covered in blood. There’s a knife in his hand. Beside him, the body of his girlfriend. Anna. Dead. He begins to scream - and then he runs.
Twenty two years later, Kierce, now a private investigator, is a new father who’s working off his debts by teaching wannabe sleuths at a night school in New York City. One evening, he recognizes a familiar face at the back of the classroom. Anna. It’s unmistakably her. But as soon as he makes eye contact with her, she bolts.
For Kierce there is no choice. He knows he must find this woman and solve the impossible mystery that has haunted his every waking moment since that terrible day. His investigation will bring him face-to-face with his past. Soon he discovers that some secrets should stay buried...