Unity:
A work of fiction or nonfiction is said to be unified when all of the parts are related to one central idea or organizing principle.

Example:
"Their yearly clothing consisted of two coarse linen shirts, one pair of linen trousers, like the shirts, one jacket, on pair of trousers for winter, made of coarse negro cloth."
-Frederick Douglass: An American Slave

Explanation:
Douglass' novel is all about the harsh and cruel treatment of the slaves, and how they should be free. This is a short example of one of the many times he he describes the living conditions of the slaves. Douglass makes a point to stay focus to his main idea of freeing the slaves, because it is inhumane and cruel to continue with slavery.