In 1865, after the end of the Civil War, Col. P. H. Anderson of Big Spring, Tennessee, wrote to his former slave, Jourdon Anderson, asking him to return to work for him. In reply, Jourdon Anderson told Colonel Anderson exactly where he could stick his offer. This letter was part of The Freedmen's Book (full download in many different formats) which was distributed to those freed after and during the Civil War, so that they would know stories of other freedmen who had done well, including Touissant L'Ouverture, Phillis Wheatley and Frederick Douglass. The book was put together and published by Lydia Maria Child, abolitionist, women's rights activist, Indian rights campaigner and all around awesome person. She became famous in her own time for her cookbook The Frugal Housewife, but today her best known work is Over the River and Through the Woods. The Freedmen's Book was part of an effort by abolitionists after the war to educate freed slaves. The American Antiquarian Society has a great website about that movement, Northern Visions of Race, Region and Reform, which has plenty of primary sources and images galore.
The Letter Repository contains hundreds of personal letters from the early 18th Century through the Second World War. A large portion of the letters are from periods of conflict, the largest chunk being from World War Two, though there are also sizable numbers from the First World War and the American Civil War. There are also quite a few love letters. You can both see scans of the letters (and photographs or other materials) as well as transcriptions, which you can edit should you spot errors. One of my favorite collection of correspondance is the one between a Herbert Beyer, who served in the Air Force in World War Two, his darling Cleo and his parents.