Laurie Halse Anderson

Separated from his friend Isabel after their daring escape from slavery, fifteen-year-old Curzon serves as a free man in the Continental Army at Valley Forge until he and Isabel are thrown together again, as slaves once more.

When her owner dies at the start of the Revolution, a greedy nephew keeps Isabel and her younger sister enslaved and sells them to Loyalists in New York, where she is offered the chance to spy for the Patriots.