The Babysitter III by R. L. Stine
Jenny is desperate to forget Mr Hagen-the maniac who terrorized baby-sitters, including her. Even when she goes to stay with her cousin Debra for the summer, she is still plagued by the same nightmares. Because Debra is a baby-sitter too, and now she is getting terrifying phone calls, just like the ones Jenny used to get...Hi Babes. I'm back. Company's coming... But Mr Hagen's dead now, isn't he...?