The Letter, the Witch, and the Ring (Lewis Barnavelt)

Blurb

A rich, magical gothic mystery from the legendary John Bellairs

Rose Rita wishes she could go to camp like her bets friend, Lewis. She's sure that boys get to have all the fun.--until Mrs. Zimmermann offers her an adveture of her own. Mrs. Zimmermann's cousin Oley has left her his farm, as well as a ring that he thinks is magic. But when the two arrive at the deserted farm, the ring has mysteriously vanished. What power does it have? And will the person who took it use the ring to do evil?