Sometimes overlooked, diplomacy was an important aspect of the American Revolution. The main countries involved were the United States, Britain, France, Netherlands and Spain. Several other countries led by Russia stayed neutral and formed the League of Armed Neutrality to protect their rights to trade and commerce.
In the end, the Americans won as much through diplomacy as from the battlefield.
Bemis, Samuel Flaff. The Diplomacy of the American Revolution. 7th ed. Blomington & London: Indiana University Press, 1967.
Bemis provides a good overview and introduction to the major diplomatic events during the Revolutionary War.
Hoffman, Ronald, and Peter J. Albert, eds. Peace and the Peacemakers: The Treaty of 1783. Perspectives on the American Revolution. Charlottesville: Published for the United States Capitol Historical Society by the University Press of Virginia, 1986.
This volume focuses on the diplomatic negotiations in 1782 and 1783 surrounding the treaty which ended the American War of Independence.
Wharton, Francis, ed. Revolutionary Diplomatic Correspondence of the United States. First. 6 vols. Washington, DC: Government Printing Office, 1889.
A comprehensive 6 volume set which provides primary sources of diplomatic correspondence associated with the Revolutionary War. There is a useful comprehensive index in Volume 1.
While spanning more than the Revolutionary period, this 4 volume set provides the complete manuscripts of all European treaties associated with the territory that became the United States. Some of the treaties are in French, Portuguese or Spanish and have to be translated by the reader into English.