The library catalog uses Library of Congress Subject Headings. The list of authorized subject headings is available in the 5-volume set at the Reference Desk. However, you can start with keyword searches to gather some relevant titles, then look at their subject headings to discover the official terms to use.
Here are some subject headings related to the final years of the Soviet Union:
Communist countries
Former communist countries
Former Soviet republics
Former Yugoslav republics
Glastnost
Perestroika
Post Communism
Russia (Federation)
Russification
Soviet Union -- Economic policy -- 1986-1991
Soviet Union -- Foreign relations -- 1985-1991
Soviet Union -- History -- 1985-1991
Soviet Union -- Intellectual life -- 1970-1991
Soviet Union -- Politics and government -- 1985-1991
Soviet Union -- Social conditions -- 1945-1991
Soviet Union -- Social condition -- 1970-1991
Soviet Union -- Social life and customs -- 1970-1991
More on Subject Searching:
Catalogers use the most precise subject heading to describe a publication. For instance, books on several of the former Soviet republics will have the heading "Former Soviet Republics," while books on Ukraine will have the heading "Ukraine" but not necessarily also have the heading "Former Soviet Republics." However, a search on both headings is likely to turn up information on Ukraine. Searching both broad and narrow headings and looking for related headings will yield better results.
Just because an item's subject heading does not include "History" as a subdivision does not mean it has no history in it. In particular, look at the publication date. It may be a source contemporary to the period, i. e., a primary source.
Even if you are doing a keyword search, using phrases from the subject headings typically improves the relevance of your results.
Browsing
After finding some sources through searching the online catalog, be sure to browse the shelves near these times. The call numbers are devised to place materials on the same subject together on the shelf. In the catalog, the call number is presented as a link, so you can even browse the shelves when searching the catalog