When doing historical research, it is important to distinguish between primary and secondary sources:
"Primary sources provide first-hand testimony or direct evidence concerning a topic under investigation. They are created by witnesses or recorders who experienced the events or conditions being documented. Often these sources are created at the time when the events or conditions are occurring, but primary sources can also include autobiographies, memoirs, and oral histories recorded later. Primary sources are characterized by their content, regardless of whether they are available in original format, in microfilm/microfiche, in digital format, or in published format." (What Are Primary Sources? Yale University)
Secondary sources are written by someone who was not present during the event or condition under discussion. Authors of secondary sources use primary sources or other secondary sources to gather their information.
Not every one of the above materials is always a primary source; each item must be considered individually.