Upvote:2
There are some letter archives from WWI:
http://www.u.arizona.edu/~rstaley/wwlettr1.htm
http://www.ourecho.com/story-1716-LETTERS-FROM-A-SOLDIER-WWI.shtml
http://soldierswills.nationalarchives.ie/search/sw/index.jsp
http://www.petergknight.com/warletters.html
http://www.canadianletters.ca/collections.php?warid=3
http://www.smythe.id.au/letters/
http://www.bookrix.com/_title-en-l-paige-wwi-letters
As another example, you can read the letters of Cicero.