They are the original source - the first time something is said, done, written, or discovered. They are the documents of something happening at the time that it happens. Secondary works like books and articles analyze and interpret primary sources.
Some types of primary documents are:
To find primary sources collected in books, do a Keyword search in WorldCat that combines the name of a person, group, or event with words such as: memoirs, diaries, correspondence, papers, personal narratives, or sources.