Primary sources serve as evidence for the interpretation of past events. Analyzing primary sources will help you to understand how an argument has been constructed and to adopt a more critical stance towards the books and articles that you encounter.
"Note is the consummate project manager, and it shows in her handbook of project management for information professionals. This book knows what it is, and what it is not; it remains true to its project scope."
Primary sources are the raw materials of history. They are documents and objects that were created during the event being studied or created later by the participant of an event, reflecting their viewpoint.