Babysitter III & IV (Point Horror 2-pack Paperbacks) by R.L. Stine

"Hi, Babes, I'm back."

Jenny the bad-luck baby-sitter is back, visiting her cousin and trying to get away from her haunting past. But her cousin is a baby-sitter too, and it looks like Jenny has brought the nightmare with her. (The Babysitter III) 

Don't live in the past. That's what they say. So Jenny's trying. Trying to forget all of the horrible baby-sitting experiences that landed her in the hospital last year. But now Jenny's in a new house. In a new neighborhood. And Jenny's new neighbor, Mrs. Warsaw, wants her to baby-sit.

Poor Jenny. If only she could "just say no" to sitting.

Because someone is watching Jenny. Waiting for her. Someone who hates baby-sitters. Someone who wants her... dead. (Babysitter IV)