Herr Mord av Dean R. Koontz.
For author Martin Stillwater, life couldn't be more perfect. He and Paige are happy together; their two daughters, Emily and Charlotte, are intelligent and healthy; and his novels are achieving long-hoped-for levels of success. So why does he feel such dread, experience sudden blackouts?
The killer doesn't know his own name, only the name he uses. He has no family, no friends, no home. He cannot recall who gives him his assignments, and he doesn't know why his targets must die. Now, however, he senses that in a town he can't yet envision, a life awaits him, a place to call home, with family and friends.
Charlotte knows something is wrong: the man moves and sounds like Daddy, but he doesn't smile as quickly or as often as usual. And when he does smile, he seems to be pretending. Daddy isn't Daddy.