Missing You by Harlan Coben

From the bestselling author and creator of the hit Netflix drama The Stranger comes the #1 New York Times bestseller about the ties we have to our past-and the lies that bind us together-as the ultimate Internet scam unfolds...
Surfing an online dating site, NYPD detective Kat Donovan feels her whole world explode. Staring back at her is her ex-fiancé, the man who shattered her heart-and whom she hasn't seen in eighteen years.
But when Kat reaches out to the man in the profile, an unspeakable conspiracy comes to light. As Kat begins to investigate, her feelings are challenged about everyone she's ever loved-even her father, whose cruel murder so long ago has never been fully explained.
With lives on the line, including her own, Kat must venture deeper into the darkness than she ever has before and discover if she has the strength to survive what she finds there.