We should also be careful in the difference and usage of primary and secondary sources of literature.
Primary sources represent first-hand accounts or direct evidence. They are created by witnesses or recorders who experienced the events or conditions being documented, and can also include autobiographies, memoirs, and oral histories recorded later.
In most instances, researchers prefer to rely on primary resources simply because they describe the original research conducted, as written by the author. In some cases, however, it’s necessary or desirable to reference secondary sources. Again, the experts at Yale (2014) provide a good definition:
Secondary sources interpret and analyze primary sources. Because they are often written long afterward by parties not directly involved . . . they can provide historical context or critical perspectives.