I read this book at some point during late grade school/junior high, probably between 4th & 8th grade, so somewhere between 1980-86. It was a pre-teen horror of the old school, by which I mean it assumes kids can actually handle seriously creepy, potentially life-threatening stuff. It was an older library book that our awesome school librarian recommended to me. Maybe published in the 50's or 60's? Definitely before the late 70's, given the age of the library's copy & when I read it. Unfortunately, there's no cover art I can reference as it had a green cloth binding with no dust jacket.
I don't remember much of the plot but what I do remember is as follows: A junior-high aged girl goes to visit her grandmother for the summer in a small village. Grandma is a witch (good, natch). Another woman has moved into town & grandma suspects her to be a bad witch. Bad witch tries to start a witch war. I can't remember if it was for a specific reason or just because she was a bad witch & that's what bad witches do. Similar in feel to The House With a Clock In Its Walls series or the Green Knowe books but a smidge darker. I believe it was located somewhere in New England, but I wouldn't swear to it. I *think* there were cars & telephones but the time frame was kinda vague. Or my memory is. And that's all for the overall plot.
I do, however, remember some weird specifics. The grandma hung a witch ball over the front door to see if the new neighbor was evil. She had a bottle tree, too. There were lots of nifty little folkloric witchy things like that. The thing that sticks clearest in my mind is that grandma gets a letter with what appears to be child's hand print. Grandma recognizes it as the actual skin of a child's hand & proceeds to place the skin inside an old Bible, which she wraps up tightly so it can't be opened because said palm skin is a curse. That scene has stuck with me ever since I read it, as one might imagine. It's pretty unique. Other than that, I can remember exactly where it was located in my grade school library but that's probably not much help. (Second from last bookcase on the left, third shelf from the top, right side, below the Nancy Drew books.)
The episode with the palm skin being tied up in a Bible does come from a Green Knowe book, “An Enemy at Green Knowe”. I remember being disturbed by this detail as a child!
I’m not sure that the other details don’t come from a different book, though, as Mrs Oldknow is not exactly a witch, although the antagonist Melanie Powers probably is; and the children Tolly and Ping who feature in the book are both boys.