Once Upon a Dark November by Carol Beach York, 1989.

Katie Allen likes her part-time job helping Mrs. Herron with her housework. One of the best parts of the job is that she gets to see Mr. Herron, her English teacher, at home. Katie has a crush on Mr. Herron, although no one but her best friend knows it.

One day, while she’s at the Herrons’ house, Mrs. Herron tells her that her cousin Martin is coming for a visit. She says that Martin hasn’t been in Granville in years, although he used to live with their aunt when he was young. Their aunt is Miss Gorley, the creepy lady who lives across the street from Katie’s house. When Martin arrives, he turns out to be pretty creepy himself. He never says very much to anyone and spends a lot of his time just staring out the window. Mrs. Herron is not happy to have him there and wishes that he would leave.

One day, Martin disappears, and the next day, Miss Gorley is found murdered in her house. Did Martin return to Granville just to murder his aunt? Where is he now, and who is that mysterious person who tried to attack Katie at her house, dressed in a Frankenstein costume? Did Katie see something that she wasn’t meant to see?  Katie doesn’t remember seeing anything important, but now she has to figure out what it was before it’s too late!

This book is not for young children.  It would be best for kids in middle school (about 12 and up).  There is murder and attempted murder, including the attempted murder of Katie, who is a child, because the murderer of Miss Gorley thinks that she knows too much.  There is also some discussion of child abuse, which was part of the motive behind Miss Gorley’s murder.  Katie did see some things that are important to the case, but their full importance doesn’t occur to her until the attempt on her own life.  People are not quite who they seem to be, and some of Katie’s initial impressions were closer to the truth than someone wants her to know.

The book is currently available to borrow for free online through Internet Archive.