There are two major purposes to this novel. First, the author is trying to depict slave life and make points about that institution. Second, she is trying to encourage people to have the same sorts of values and strengths that Tubman showed in her own life.
This is a historical novel about slavery. In the book, the author is trying to show us that slavery was a horrible and unjst institution. She feels that most textbooks that students learn from do not do a good enough job of conveying this fact. Therefore, she wanted to write this book as a way of showing people how bad slavery really was.
At the same time, she wants to highlight Tubman's qualities. She wants readers to see that Tubman was courageous and caring and that she never gave up. These are qualities that would be useful for all people and Petry is encouraging us to learn these qualities from Tubman's example.