This was another one of those paperbacks I borrowed from a girl in middle school, around 1992 or so. The main character was a recently divorced woman moving into an apartment. I think that the ex had been abusive in some way and she'd obtained a restraining order. Initially, things go well for her. The neighbors are friendly, particularly a kind of geeky guy nearby her. But after a while, she starts receiving packages from a stalker who claims they're always watching her (at one point, one of the letters contains either lyrics or a recorded version of Sting's "Every Breath You Take", which she found kind of silly, and started making up other lyrics including things like "Every cake you bake", etc), someone keeps smashing in her windows with no trace of how, she's got a feeling of extreme dread and the presence of something otherworldly when she visits the laundry room, and she finds the stray kitten she adopted dead, with its eyes clawed out. I remember that she suspected her ex for a while, but something happened that proved to her that it could not have been him.
I do remember a decent amount of the ending:
It turns out that the geeky neighbor is the one stalking her. He'd devised some sort of sonic weapon, which he was using to try to make her scared enough to run to him. The shattered windows were the result of high frequencies and the feeling of an otherworldly presence was the result of subsonics. I want to say that he indicated that the kitten had been a mistake as he hadn't realized what it would do to a creature with more sensitive hearing.