It should be particularly noted his two-volume work on Balkan history from Bulgarian perspective, published in Lewiston, N.Y., Edwin Mellen Press, 1993; two-volume history called "Europe in the 20th Century" published in 2002-2003 and a book on "The World of Megamyths: Some Political and Historiographical Myths of the 20th century" in 2008 (New Bulgarian University Press).

He authored also a series of books on the medium-sized and small countries in the European policy on the eve of World War II as well as a monograph titled "Are the Bulgarians Slavs?"

He has given lectures and papers in various universities and research centers in Austria, Bulgaria, England, Greece, Hungary, Italy, The Netherlands, Poland, Romania, Russia, Turkey, and the USA.

[citation needed] Tzvetkov was member of Union of Scientists in Bulgaria (since 1991), the American Historical Society (since 1991), of the Academy of Political Science, New York (since 2007).

[citation needed] For this reason Prof. Tzvetkov's family donated to NBU his private library along with his scholarly and personal archive.