The Letter, the Witch, and the Ring by John Bellairs

Rose Rita Pottinger is about to have a horrible summer.

Her best friend, Lewis, is going away to boy scout camp, leaving Rose Rita (a girl, and thus not allowed to go along) alone all summer with nothing to do and no one to play with. Rose Rita feels betrayed and angry, but there's nothing she can do about it.

Then a friend offers an alternative to a boring summer: Mrs. Zimmerman has just gotten a letter informing her that her cousin has left her a farm up near Petoskey, and she invites Rose Rita to come with her to look over the farm, sign some papers, and then spend a few weeks driving around the Upper Peninsula.

But there's more to it than just a road trip, you see, for Mrs. Zimmerman is an honest-to-goodness witch, and her cousin has left her not only the farm but also a magic ring he discovered. Unfortunately, when Rose Rita and Mrs. Zimmerman arrive at the farm, they find that it has been ransacked, practically torn to pieces, and the ring is gone.

And then strange things start happening... strange sounds, faces appearing, shadows in the night... and Mrs. Zimmerman begins to act oddly. Very oddly indeed...

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P.S. There are actually two books that take place before this one which you might want to read first: The House with a Clock in Its Walls and The Figure in the Shadows.