As a first book, my general recommendation always goes to Charles Adams, When in the Course of Human Events. He's a northern historian. Many of the books by southern authors come across as very emotional which undermines their credibility even though many citations are provided.
Citations are a HUGE part of the reason that I recommend the Adams book though. He cites sentences mid-paragraph while several of the others will just list a collection of cites at the end of a chapter making it harder to figure out what came from where. With the Adam's book, when you read a sentence that makes you say "There is no way that's real" and you can punch the cite straight into Google to either find it from their documents archive or look it up in the Library of Congress and say...wow...holy crap that is real.
If you want to get some initial inspiration though, the
Cherokee Declaration of Causes documents so much of what was going on 6 months into the war that most people never hear about, it's fairly eye opening.
From there go to McPherson's Battle Cry of Freedom. That particular tome is the core reference that most people recognize as the best documentation of the time period. Reading Adams first will let you spot the critical pieces that are left out or glossed over.