What is the author’s purpose or the main idea of Chains?

Chains is a young adult historical novel by Laurie Halse Anderson, published in 2008. The author's purpose, or the main idea of the book, is to teach the reader about the institution of slavery and how it operated in the backdrop of the late Colonial era on the brink of the American Revolution.
The reader learns about slavey primarily through the experiences and life events of Isabel, the thirteen-year-old protagonist, as well as through excerpts of historical records and famous writings of the era woven into the work. Isabel is a young slave who is owned by a Loyalist family, the Locktons, by whom she is treated very poorly. She eventually agrees to assist the cause of the Revolution by spying on them.
The author also wants the reader to see the contradictions of the times, as the fight for freedom from the British crown did not include a liberation for the slaves from their physical and societal chains.