"The Civil War: A Narrative" in its 3 volume entirety is the standard for me. Some may be intimidated by the daunting task of beginning such a massive read (almost 3,000 pages), but I can assure you that after just the first few pages it stops being about simply reading and becomes more of a frenzied desire to consume all 1.2 million words.
Anything by Bruce Catton deserves a look in my opinion, he set the standard in the 1950s/1960s that is still what modern writings on the subject are measured to this day. A personal favorite of mine is also "Gettysburg" by Stephen W Sears. That book stands a full head and shoulders above "The Killer Angels", no other historic account of any event is as easy and enjoyable to read as Sears' work. Sears also penned "Landscape Turned Red" which is based around the Battle of Antietam/Sharpsburg, it's very good as well.
Avoid anything by James McPherson like the plague. This is obviously just a suggestion based on opinion, but seriously the guy is awful.