Harper Collins Dictionary of American Government and Politics. Lots of useful, recent entries including a chronology of the Gulf War and all four verses of the National Anthem. International Encyclopedia of the Social Sciences. Long survey articles on all phases of the social sciences. Includes contributors and bibliographies.
Emphasis is on principles, concepts, theories, methods, etc., rather than findings of research.
I5 Political Parties and Civic Action Groups. 1981 Dated, but still a good resource for historical information. An excellent subject encyclopedia which discusses people, issues and concepts. The bibliographies are particularly useful. Safire's Political Dictionary.