Magic Elizabeth by 

Norma Kassirer

The other side of the mirror...

It looks like it's going to be a bad summer for Sally: She's got to stay with her creepy Aunt Sarah while her parents are away. Trapped in a spooky old house and stalked by an unfriendly cat, Sally begins to explore. She sneaks upstairs and discovers the attic, where she finds herself drawn toward an old mirror. When she looks into it something magical happens...

Sally finds herself in the past on the other side of the mirror where another little girl--also names Sally--has lot a beloved doll, Elizabeth. Sally searches and searches, but the doll seems to have vanished. Can the two help each other across time?

This is the book that did it for me.  I was in the second grade.  Came home from school with one of those Scholastic Book Club order forms.  My mom said "Donna, pick a book. You need to read more."  I chose this book.  I was hooked. To this day I still have a copy of this book.  When I feel nostalgic, I re-read the story.  It brings back wonderful memories of my childhood.