Oh, the Keegan book is shockingly bad, don't let yourself be put off by this.
If you are interested in the tactics, Paddy Griffith's
Rally Once Again: Battle Tactics of the American Civil War is still valid and a good read. There are newer studies by Nosworthy and Earl Hess, though, which are very good but a bit dryer.
For very readable battle accounts, I can recommend Noah Andre Trudeau, e.g. his book on Gettysburg.
And one of the best studies on the soldier's experience, and one of the best books on the war in general, is
The War for the Common Soldier: How Men Thought, Fought, and Survived in Civil War Armies by Peter Carmichael.
If you're interested in a specific topic, feel free to send me a PM, I'll be happy to recommend you some books
