Tell No One by Harlan Coben

Tell No One is a fast-paced psychological thriller following Dr. David Beck, a pediatrician grieving his childhood sweetheart and wife, Elizabeth, who was brutally murdered by a serial killer eight years ago. David's numb existence is shattered when he receives a mysterious, encrypted email containing a message and a video link that only Elizabeth could know, accompanied by the strict warning: "Tell no one." As David desperately tries to discover if his wife is still alive, the police suddenly reopen the case and suspect him of murder, forcing him to go on the run from both the law and ruthless, shadow-dwelling killers who will stop at nothing to keep the dark secrets of that night buried.