The focus here is online dictionaries and encyclopedias (reference sources)
A few key works may still be available in print, but these tend to be older.

Bevir, M. (Ed.) (2010). Encyclopedia of political theory. Sage Publications.

Brown, G., McLean, I., & McMillan, A. (Eds.). (2018). A Concise Oxford Dictionary of Politics and International Relations. Oxford University Press.

Denemark, R., & Marlin-Bennett, R. (Eds.). (2010). The International Studies Encyclopedia. Wiley-Blackwell.

A collection of academic sources, useful for defining key terms and getting short topic overviews.

Browse the politics titles (under social sciences) or search across the complete ORO corpus for insights from a range of disciplines.