Thing at the Door — Henry Slesar

When Gail Gunnerson was six years old, a Thing came to her bedroom door. It was so horrifying that her shocked young mind refused to remember what it was. Even after twenty years, the specter of her childhood is totally forgotten, buried beyond recall.

But Gail still has nightmares that bring her awake screaming. And then the nightmares become reality. There are noises in the night. She sees the corpse of her mother dangling from an attic rafter.

Her psychiatrist says it's all in her mind.

The private detective says it's because of her money.

And both know that Gail will die unless the Thing as the door returns!