The Length of a String by Weissman, Elissa Brent-Hardcover

Imani is adopted, and she's ready to search for her birth parents. But when she discovers the diary her Jewish great-grandmother wrote chronicling her escape from Holocaust-era Europe, Imani begins to see family in a new way.

Imani knows exactly what she wants as her big bat mitzvah gift: to find her birth parents. She loves her family and her Jewish community in Baltimore, but she has always wondered where she came from, especially since she's black and almost everyone she knows is white. Then her mom's grandmother--Imani's great-grandma Anna--passes away, and Imani discovers an old journal among her books. It's Anna's diary from 1941, the year she was twelve and fled Nazi-occupied Luxembourg alone, sent by her parents to seek refuge in Brooklyn, New York. Anna's diary records her journey to America and her new life with an adoptive family of her own. And as Imani reads the diary, she begins to see her family, and her place in it, in a whole new way.