American Girl: Addy #1: Meet Addy book by Connie Rose Porter

Addy Walker's family is planning a dangerous escape
from slavery in the summer of 1864. But before they can make the escape, the worst happens--Master Stevens decides to sell some of his slaves, including Poppa and Addy's brother, Sam. Addy and Momma take the terrible risk of escaping by themselves, hoping that the family eventually will be together again in Philadelphia. Set during America's own struggle over slavery, the Civil War, Addy's story is one of great courage and love--love of family and love of freedom.