Assuming you're referring to the US Civil War, the best single-volume history still on the market is probably James McPherson's Battle Cry of Freedom. It does a good job of setting up the conflict with a lot of the relevant context, and provides an overciew of the war itself that quite nicely balances detail with readability. It's actually only one volume in the Oxford History of the US series, but it stands out as one of the better volumes. The book is both a widely-read popular classic, and commonly cited in scholarly work on the subject.