As the story is set during the American Civil War and its aftermath, and the protagonists are African Americans who begin the story as slaves and end up as free blacks, the key issue you should discuss is racism. In the novel, relationships between whites and blacks are treated in a balanced fashion. The author dramatically portrays the horrors of slavery, and of the racism the black family endures in the south after the Civil War. Vyry and her family are raped and beaten as slaves and persecuted later in the novel by the Ku Klux Klan.
Despite this, the redeeming moments of the novel are ones where individual love and generosity triumph over racial conflict, in Vyry's attempts to care for white people who are suffering from adversity and the white people such as Randall Wheelright who help blacks out of a sense of justice. It is when Betty-Alice Fletcher and Vyry step outside the racial conflict of the period to help each other as women that we final get the happy ending to the novel.
Thus for your essay, the societal ill is racism and the solution is kindness and generosity to those in need without regard to race.