The Ghost in the Mirror

Mrs. Zimmermann hasn't regained her powers from her last encounter with dark magic, and Uncle Jonathan and Lewis have left for the summer to travel, leaving her and Rose Rita bored and lonely at home. That is until Mrs. Zimmermann sees visions and shadows of her old magic teacher, Granny Wetherbee, trying to contact her from beyond the grave. Mrs. Zimmermann and Rose Rita set off to Pennsylvania to solve the mystery and in doing so travel back in time to Granny Wetherbee, who tells them that they must save a good wizard who is destined to die for a wrongdoing he didn't commit. 

John Bellairs (1938-1991) was an award-winning American author of many gothic mystery novels for children and young adults, including The House with a Clock in Its Walls, which received both the New York Times Outstanding Book of the Year Award and the ALA Children's Books of International Interest Award.