Mirror of danger = originaly published as Come back, Lucy

Originally published in Britain as Come Back, Lucy.

The mirror looked ordinary. It had a heavy gold frame and leaned against the wall in the attic. But when Lucy first looked into the mirror, a strange thing happened. Another girl's face appeared beside hers, laughing. Whirling around in surprise and alarm, Lucy saw a girl wearing a long dress, a pinafore...and a sly triumphant look on her face. Also, the attic had disappeared and another room had taken its place - a room that belonged to the past.

The girl in the mirror was Alice, and she had lived a hundred years ago. Lucy found she could visit her any time she looked into the mirror. At first, it was fun to enter the past and see how Alice had lived. But, gradually, the chill of terror took over. Alice's friendliness turned into a sinister desire for power and control over Lucy. And Lucy found it harder and harder to resist being trapped in the past with Alice forever!