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Governments produce a wide array of documents ranging from Congressional or Parliamentary debates and reports, census data, agency or departmental papers to brochures, maps and posters. As primary sources these official governmental publications provide insight into all aspects of history from social issues to politics, from cultural mores to scientific endeavor
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 homefront. 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 Unpublished diaries and letters are found in manuscript repositories such as the Special Collections here at UNLV. Help:Making Sense of Letters & Diaries
Oral histories provide a personal angle on history and often gives voice to the common person. However it important to keep in mind that oral histories are subjective and individualistic and that memory is fallible.