A leading authority on the life and presidency of Abraham Lincoln, Phillip Shaw Paludan was a professor of Lincoln Studies at the University of Illinois Springfield.
This is an excellent overview of the political, military, economic, and diplomatic issues that suffused the Civil War. While it may not go into great detail about any one issue, it provides a through account of how these issues combined to form the Union's response to secession and war. Paludan moves easily between elite stories (e.g. Lincoln, Seward, Grant, etc) and the experience of 'average' workers, soldiers, or farmers.