Primary sources are created at the time of an historical event or during a particular time period.  In literature, they often includes a writer's personal papers, drafts, correspondence, etc. They can also include:

• Diaries, journals, interviews, letters, memos, manuscripts.
• Memoirs and autobiographies.
• Records collected by government agencies, such as births, deaths, marriages; permits and licenses issued; census data, etc.).
• Records of organizations connected with a writer.
• Published materials (books, magazine and journal articles, newspaper  articles) written at the time about a particular event.
• Photographs, audio  and video recordings, and film.
• Artifacts of all kinds: physical objects, buildings, furniture, tools, appliances and household items, clothing, toys.

• Guida generale degli archivi di Stato italiani (4 vol.). 
• Le fonti archivistiche : catalogo delle guide e degli inventari editi (1861-1991). 
• Guida alla ricerca d'archivio. 
• Inventory of Western manuscripts in the Biblioteca Ambrosiana.  
• I codici Petrarcheschi della Biblioteca Vaticana.
• Dictionary of Archival Terminology (with Italian equivalents). 
• Finding archive catalogs or guides in the Libraries Catalog: do a Keyword Search on Subject keywords such as archives, guide, archivi nazionale, italy, manuscripts, catalogs, handbooks. Another good subject keyword is Sources. This gets you primary literature. There are many indexes and guides to individual archives, or archives in specific subjects areas, such as music, history, literature, etc.

Archive Finder: Indexes US and UK archives

ArchiveGrid: A database for locating special collections worldwide.

WorldCat: Limit your search to "Archival materials". Manuscripts back to the Middle Ages

• Working with primary sources sets your research and writing apart from others'.
• Grants are often focused on research using archival sources.
• Manuscripts (of all kinds) do not yield their meaning easily. It's up to the researcher to make sense of them.
• Often what you find is not what you were looking for. Be awake to new discoveries!