The spirit of laws, by Charles de Secondat, baron de Montesquieu. [Translated by Thomas Nugent, rev. by J. V. Prichard]
On the origin of inequality. On political economy. The social contract. By Jean Jacques Rousseau. [Translated by G. D. H. Cole]
Suggested terms to look for include - diary, diaries, letters, papers, documents, documentary or correspondence.
Combine these these terms with the event or person you are researching. (example: civil war diary)
Also search by subject for specific people and events, then scan the titles for those keywords or others such as memoirs, autobiography, report, or personal narratives.
An additional option is to do a search on a topic and use the "Resource Type" limiter on the left side of the results and choose the option "Primary Sources" - please note this is extremely limiting as most primary source materials are not marked in records with this resource type and you will miss seeing a large number of useful items.
Off campus access instructions (for e-books)
De l'esprit des lois by 