What you call "historical records" are actually very diverse sources. Any campaign or battle would generate many orders and reports, and a significant number of those were later ordered saved by Gen. Henry Halleck and printed as the Official Records of the War of the Rebellion. It's online now, and you can browse it. But as well as the Official Records, there were many memoirs written- sometimes sooner, sometimes later. Not only from soldiers, officers, but civilians. And some would be asked to write about their experiences- The Century magazine solicited first person accounts of the War from participants on both sides- especially those the editors knew might have an axe to grind or a complaint.

There were often quite a lot of details preserved- this was a war, after all. A soldier at Shiloh would likely recall a lot of his two eventful days in it, and if Gen. David Hunter burned your fine house in the Shenandoah Valley, you generally remembered it in detail. But the fact that it was a war also means that written records are not complete. For example, for the defense of Little Round Top at Gettysburg, Col. Joseph Chamberlain became very famous ( and later a central character in Michael Shaara's The Killer Angels) in part because he lived to write and tell his story: other Union commanders there ( Strong Vincent, Paddy O'Rourke , Stephen Weed and Charles E.Hazlett) were killed, and couldn't.