Who is Mrs. Finch?

Sarah sat at her desk quietly, looking at her reflection in a small mirror. It was her...but not. Her blond hair was mixed with white. Her figure was not what it once was. But the oddest thing was her thoughts...when she thought about being a teenager, the memories were foggy...they hadn't changed...but it was if they had been, like her...aged by decades in a flash. And she knew virtually nothing about Mrs. Finch's life outside of the library. She began to pace, absent mindedly shelving some books as she did.