Underground Railroad is a great book and easy read. I will probably be put on my read again list!
I first would say first the character development is top notch. Given the number of characters, I was not expecting that all characters would be fully developed.
Underground Railroad is fiction, but still shows the evils of slavery, the slaves' dreams and actions to become free. The plantation dynamics were demonstrated by the actions of all the masters.
All and all I'd give it a solid 4/5.