The Grownup by Gillian Flynn

She's making a decent enough living faking her way through being a psychic with some soft-core sex in the back, albeit illegal. Then comes Susan Burke—scared and desperate, believing her house is haunted by a malevolent spirit that may or may not be possessing her stepson—and she sees a good opportunity for money. However she can feel it when she walks into the Carterhook Manor and when she meets the stepson—she feels it watching her.