What are Primary & Secondary Sources?
Primary sources are the main text or work that you are discussing (e.g. a sonnet by William Shakespeare; an opera by Mozart); actual data or research results (e.g. a scientific article presenting original findings; statistics); or historical documents (e.g. letters, pamphlets, political tracts, manifestoes). ["What is a Source?" Recognizing and Avoiding Plagarism. Cornell University. College of Arts and Sciences.]
Secondary sources are "books, periodicals, web sites, etc. that people write using the information from primary sources. They are not written by eyewitnesses to events, for instance, but use eyewitness accounts, photographs, diaries and other primary sources to reconstruct events or to support a writer's thesis about the events and their meaning. Many books you find in the Cornell Library Catalog are secondary sources." [Finding Primary Sources: A Web-Based Guide]
Finding Primary Sources
Here are two excellent guides to finding primary sources: