Some of the Robert Kennedy's paper have been released online by the John F. Kennedy Library & Museum. Currently (10/2012) material covers the Cuban Missile Crisis.
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.
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Off campus access instructions (for e-books)
The Gospel According to RFK: Why it Matters Now by 
To Seek a Newer World by 