What is Ann Petry's purpose for writing Harriet Tubman: Conductor of the Underground Railroad?

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.