The Devil's Arithmetic by Jane Yolen
Hannah dreads going to her family's Passover Seder — she's tired of hearing her relatives talk about the past. But when she opens the front door to symbolically welcome the prophet Elijah, she's transported to a Polish village in the year 1942, where she becomes caught up in the tragedy of the time.
Such a small book, but so powerful; it defies explanation. I can't believe that I didn't read this book sooner.