Battle Cry of Freedom: The Civil War Era, by James McPhereson is the sixth book in the Oxford History of the United States, and an excellently concise work on both the military and political sides of The Civil War. Since this is the Civil War we are talking about, "concise" means a little bit short of 900 pages.
Those who have read much military history and political history know that they are very different genres. Much military history can get into very detailed descriptions of manuevers and skirmishes on very small levels, and often gives a bewildering list of unit names and numbers, often accompanied by unclear maps. Writing truly clear and engaging military history is a rare talent. McPhereson's military writing is fairly good, but his political history and discussion of the economic and social background of the war are excellent. He discusses many things that are needed to understand why and how the Civil War broke out: including the disastrously incompetent, corrupt and treasonous administrations of Pierce and Buchanan. He also explains the gradual change of the North from fighting a limited war with the goal of reunification, to a more revolutionary war against oligarchy of the South and the slavery they depended on.
Another good advantage of this book is that it was written by a writer who is obviously Pro-American. Many Civil War histories seem to be written by military experts whose sympathy with the romantic image of the South makes them blind to the many faults of the South. McPhereson is not at all sympathetic to the slave holding oligarchy that launched the Civil War, and points out their plots and schemes before and during the war, as well as their often incompetent handling of the war, and also the crimes they committed during the war. It is information that Americans need to know.
If you want to learn more about the Civil War, and you really should if you want to understand the American society of today, this is perhaps the book you should start with.