Nobody's Fool by Harlan Coben

 While Sami Kierce is backpacking through Europe before starting med school, he meets a beautiful girl named Anna in Spain and they begin a short but meaingful fling. One morning, he wakes up and finds Anna in bed next him covered in blood and he is holding a knife.  He calls the cops then he calls his father, who tells him to get on the next plane to America. Years later, Sami is an ex-cop, happily married, and teaching a criminology class and doing investigating for hire. One night a woman drops into his class --who looks strikingly like an older version of Anna from Spain. He chases her and lands on the estate of the wealthy Belmond family, which became famous when their beautiful daughter Victoria disappeared on New Year's Eve 1999. Sami is determined to find out if Anna and Victoria are somehow related. At the same time, a convict named Tad is released from prison--for killing Sami's fiancee at the time, Nicole, a fellow cop. Then soneone starts following Sami's wife, Molly, and sending lewd texts to Sami about his wife. 

Sami was a good cop (till he got thrown off the force) so he eventually untangles the Victoria mystery, the Anna mystery, and whether or not Tad really killed Nicole and what he's been up to since his release from prison. It's a good story with lots of twists and turns--with a believable hero who's had his ups and downs but is still trying to do the right thing.