These are some of the terms you might use to find primary sources:
diary or diaries
Subject headings are a formalized way of of searching library catalogs. The right subect heading can help you get to information more quickly.
For example, World War I can be described using the following terms:
The Great War
The First World War
World War One
World War 1
World War I
You could search the library catalog for all those keywords. Or, use one subject heading to find all of the library's materials about World War I:
World War, 1914-1918
Who determines subject headings? The Library of Congress. A handout outlining the Library of Congress Subject Headings is available at the Olin Library Reference Desk.