Best way to learn about the American Civil War is to read histories of the American Civil War. Fiction about the Civil War won't tell you a lot about who actually did what, when, how, and with what result. It might give you snippets here and there, but will mostly go off on tangents about some fictional story set during the American Civil War. On the other hand, a good history of the war will give you lots of really good context and background for any Civil War fiction you read afterwards, and give you good insights into what that fictional story has to add to (or subtract from) the story of what actually happened.

The same is true for any history about any subject. Similar things are true for biographies. Even textbooks can be enlightening.