The Ghost in the Mirror (Lewis Barnavelt #4)

It was a warm summer day in 1951 when Rose Pottinger and Mrs. Zimmermann entered the tunnel. When they had emerged, it was snowing...and the year was 1828.

Mrs. Zimmermann had felt that the ghost of Granny Wetherbee, who had taught Mrs. Zimmermann witchery, was in trouble and needed help. So she and Rose Rita had traveled to Pennsylvania where Granny had lived. They never dreamed that they would also journey back to a time long ago where they would encounter a sorcerer more terrifying than either could have imagined.