The Ghost in the Mirror — John Bellairs

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.
 
Books by John Bellairs:
 
The Doom of the Haunted Opera
The Figure In the Shadows
The Ghost in the Mirror
The House With a Clock In Its Walls
The Letter, the Witch, and the Ring
The Mansion in the Mist
The Specter From the Magician's Museum
The Treasure of Alpheus Winterborn
The Vengeance of the Witch-Finder