The Letter, the Witch, and the Ring [B0605]

1976 PB. Rose Rita Pottinger is dreading summer. With her best friend, Lewis Barnavelt, away at Boy Scout camp, vacation threatens to be altogether boring. But when Mrs. Zimmermann, Lewis's next door neighbor and a genuine witch, receives a strange deathbed letter from an eccentric uncle, unexpected things start to happen. Rose Rita and Mrs. Zimmermann set off on a trip to discover the meaning of the letter. A ransacked farmhouse, a missing ring, shadowy figures appearing in the night, and mysterious magic symbols are just the beginning as they are gradually drawn into a terrifying world of occult mysteries, where Mrs. Zimmermann's failing powers can't help them.

From Amazon/GoodReads reviews: "A kid's book from the 70's dealing with magic and non-binary gender...interesting."; "Empowering tale for young/adolescent women."; "A kids fantasy adventure with witches and forests and adventure with a side of gender studies. I don't usually like when men write the female voice but I think this really hit a few things on the head and aged well."; "Out of all the John Bellairs' books I remember reading this one is the scariest. I didn't read it as a child, but reading as an adult still creeped me out a little and it was an interesting look at Rose Rita and gender conformity, especially given the decade it was published in."