Many printed/published documents such as books, government documents, maps, films, etc. are primary, e.g. are first hand accounts. Library catalogs help identify such sources. Interlibrary loan (ILLIAD) services can retrieve them.
[Topic, e.g. United States > History > Civil War, 1861-1865 >] Sources
Recommended by Nathaniel Philbrick in an interview New York Times, May 26 2016:
"What was the most interesting thing you read in researching your new book?
Some magazines are digitized in Google Books, lncluding Life Magazine (digitized and keyword searchable from 1935-1972, no printing.) Use the link to browse a list of other titles available. Or, do an advanced search of Google books, limiting to magazines, notice you can also select a date range. Mostly articles from U.S. during 20th century
JStor includes historical sources.. For example currently in JStor American Naturalist is available from 1867-2015, American Scholar runs 1932-2011 and American Sociologist 1965-2013. Some JStor journals date to the 19th century, many started in the early 20th.