Below is a selected list of online databases which can be useful for finding secondary literature.
Journal Articles
America: History and Life (1954 to present) Abstracts articles and provides citations to reviews and dissertations on the history and culture of the US and Canada from prehistoric times to the present.
JSTOR provides full-text access to thousands of articles from archived academic journals. Date coverage varies by individual journal title, but a typical coverage range is from the beginning of the journal until 5 years ago.
Google Scholar Provides a simple way to broadly search for scholarly literature.
Humanities Abstracts (1984 to present) indexes around 500 English language journals in such fields as history, philosophy, literature, and archaeology.
Web of Knowledge indexes thousands of academic journals. Date coverage for social sciences titles is 1956 to present; humanities titles are indexed from 1975 to present.