Greetings Cosmic Americans!
It is indeed a great boon to Civil War historians that mid-nineteenth century Americans were - for the most part - a literate society. Census information from 1850 and 1860 suggests that somewhere between 75% and 90% of adult whites were...that's right...literate. Literacy rates were higher in the North, and higher again in urban areas. But in terms of the general population - most white Americans at the beginning of the Civil War could read and write.
Of course, there are degrees of literacy. Reading a tavern sign does not make one a man of letters, as it were...and I have read letters written by people who were just barely hanging on to what we might term "literate." But for the sake of argument, let's just say that we are dealing with a literate society.
This is useful information for two reasons. One: citizens read things - and the technological onslaught of the printing press and the railroad meant that reading materials were disseminated far and wide...to every corner of the nation. Newspapers reached millions - and so did books. By 1860, the good people of the United States were pretty up to date on the issues (Uncle Tom's Cabin, anyone?). So - those who supported and enlisted to fight for their respective causes knew what was at stake. Don't let anyone tell you differently. Two: people wrote everything down. Whether in letters home or in diaries and journals, soldiers and civilians recorded their thoughts, their actions, their opinions...what have you. And thus we now have at our disposal a wide range of testimony from all classes, ethnic groups, and so on.
Now that is some pretty good news. Oh sure, it can sometimes be a bit overwhelming. I mean, where do you stop? There is so much extant written material that it would be near impossible for an army of historians to ever get through it all. But let's rejoice anyway. Our nineteenth-century friends had the good sense to write down what they thought - it just makes figuring them out a little easier.