Mr Murder - Dean Koontz

Because when Christmas comes this year,
because there's something new to fear.
Santa's twin—who is evil and mean—
stole the sleigh, will make the scene,
pretending to be his good brother.
Guard your beloved children, mother!
Down the chimney, into your home,
here comes that vile psychotic gnome!

For author Martin Stillwater life couldn't be more perfect. He and Paige are happy together—absurdly happy considering how many of their friends are divorced, separated or cheating on each other—and their two daughters, Charlotte and Emily, are intelligent, well-adjusted and healthy. At last, Martin's novels are achieving long-hoped-for levels of success, with the two most recent paperbacks being bestsellers. And, though he doesn't like publicity, he's about to be in People magazine. So what's going wrong?
Why does Martin feel such dread, experience sudden blackouts? There must be a simple explanation. Mustn't there?

The killer doesn't know his name, only the names 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. He has no past memory and no knowledge of what he will do before he does it. He does not know who he is. Sometimes, in despair, he is overwhelmed by his loneliness, and tortured by the meaninglessness of his life. Then he sobs as if a child. Now he's reached a turning point, is trembling on the brink of an insight that will change his life: 'I need to be… to be… I need to be someone.' And he senses that beyond the horizon, in a town he can't yet envision, a life awaits him, a place to call home, with family and friends.

Charlotte knows there is something wrong and is troubled as she blinks back her tears. The man stands, sits, 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.