Letters and diaries provide a personal angle on history and often gives voice to the common person such as the WWI soldier in the trenches or the sweetheart on the home front. However keep in mind that, like all sources, letters and diaries follow the convention of their times and must be considered in light of the writer's motivations and biases. There are a number of published collections of letters especially those written by well-known historical figures (Walt Whitman or Winston Churchill). Unpublished diaries and letters are found in manuscript repositories such as the Special Collections here at the UW.
To find published letters and diaries available at the UW Libraries use the search terms correspondence or diaries in UW Libraries Search. For example: