Dictionary of National Biography article on Churchill: http://www.oxforddnb.com/view/article/32413
Winston S. Churchill. Randolph S. Churchill; Vols. 3- by Martin Gilbert. London: Heinemann, 1966- [includes very helpful "Companion documents" volumes.
Firestone DA566.9.C5 C47 1966
Entries on Winston Churchill in a variety of subject encyclopedias
Annotated bibliography of works about Sir Winston S. Churchill. Curt J. Zoller, [editor]. Armonk, N.Y.; London: M.E. Sharpe, c2004.
Firestone Library (F) Z8169.45 .A56 2004
Historical dictionary of Anglo-American relations. Sylvia Ellis. Lanham, Md.: Scarecrow Press, 2009.
History Reference (SH) E183.8.G7 E45 2009
A Companion to Early Twentieth-Century Britain
A Companion to Contemporary Britain 1939–2000