Slavery is the big theme in Douglass's Narrative, since he wrote his book to convince people that slavery was wrong. You might call the book anti-slavery propaganda, but we don't think Douglass would see anything wrong with that: for him, the important thing was that everything he said about slavery was true. And since he had plenty to say on the subject, the book remains one of most important historical documents from that period, showing us the kinds of lives that American slaves like Douglass lived.
In his autobiography, Douglass describes two different kinds of freedom: while legal freedom can be given or taken away by the government, personal freedom is something that comes from within.
Douglass argues that slaves aren't the only victims of slavery. Perhaps because he's writing a book for a mostly white audience, he focuses on how slavery corrupts every American who takes part in it.