Beach Blanket Psycho

Horror mystery novel. Terri and her friends have rented a beach house for the summer. Terri's even invited her pen pal, Ben Marsh, whom she's never met. When Ben shows up, he charms everyone. But Terri thinks there's something strange about Ben. He doesn't seem like his letters at all. Then some scary things start happening. Could Terri have invited a psycho to the beach house?