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Soviet Union -- Bibliography of bibliographies
Bibliografiia russkoi bibliografii: ukazatelʹ bibliograficheskikh posobii. Eds. S.N. Kotlomanova et al. St. Petersburg: Rossiiskaia natsionalʹnaia biblioteka, 2000-<2014>. Part 4 (1928-1933), issue 1-2 is available in digital format.
Istoriia SSSR: annotirovannyi ukazatelʹ bibliograficheskikh posobii, opublikovannykh na russkom iazyke s nachala XIX v. po 1982 g.: v dvukh chastiakh. Eds. G.A. Glavatskikh, N.V. Kadushkina, L.M. Maslova. Moscow: Gosudarstvennaia biblioteka SSSR imeni V.I. Lenina, 1983-1984.
Viola, Lynne, and Sheila Fitzpatrick. A Researcher's Guide to Sources on Soviet Social History in the 1930s. Taylor and Francis, 2016.
Bibliographic Research Guide to Soviet History. Compiled by Andrea Graziosi, University of Naples. Website, updated 2004. Copyrighted by Presses Universitaires de France. Any use of this bibliography for purposes other than teaching and research is prohibited.
A Research Guide to Soviet History. Compiled by Donald Raleigh, University of North Carolina. Website. Wayback Machine internet archive, June 20, 2003. (Organized by document type ("Bibliographies," "Dissertations," "Periodicals," etc.). References to "Davis Library" are to the main UNC library, not the Davis Center Library at Harvard.)
National Biographies, created by the International and Area Studies Library at the University of Illinois Urbana-Champaign
Uzbekistan (another fully searchable version here)
Digital Handbook for Research on Soviet History
The Yeltsin presidential library
Europeana (combined holdings of thousands of European archives, libraries and museums):