Nobody's fool by Harlan Coben

In Harlan Coben's Nobody's Fool, a disgraced former NYPD detective, Sami Kierce, is haunted by a dark secret from his past. After an unsettling encounter in Spain as a college student leaves him covered in blood and with a dead girl, he runs. Years later, a woman in his criminology class triggers his memory, and he's determined to solve the mystery, even if it means confronting his past.