The War That Saved My Life by Kimberly Brubaker Bradley
From Penguin Random House:
Ten-year-old Ada has never left her one-room apartment. Her mother is too humiliated by Ada’s twisted foot to let her outside. So when her little brother Jamie is shipped out of London to escape the war, Ada doesn’t waste a minute—she sneaks out to join him.
So begins a new adventure for Ada, and for Susan Smith, the woman who is forced to take the two kids in. As Ada teaches herself to ride a pony, learns to read, and watches for German spies, she begins to trust Susan—and Susan begins to love Ada and Jamie. But in the end, will their bond be enough to hold them together through wartime? Or will Ada and her brother fall back into the cruel hands of their mother?