Internet History Sourcebooks Project
Fordham University: Modern History Sourcebook: United Nations and Ussr
This site from Fordham University contains actual letters between the United Nations and the USSR. These letters are attempts to reach a resolution in the Cuban Missile Crisis.
Other
Cuba Net: Marine Officer Wrote After Bay of Pigs
In the after-action report filed by Marine Col. Jack Hawkins following the Bay of Pigs, he writes that the only way to oust Castro would be with another attempt using overt intervention with U.S. military power.