Subject encyclopedias are a great resource for finding background information. You could think of them as books filled with detailed articles written by scholars in various disciplines that are like well-written research papers.
They are not your average general encyclopedia, like World Book, that has brief articles on a wide variety of subjects.
Subject encyclopedias are:
Examples:
10,000 Ideas for Term papers, Projects, Reports and Speeches by 
Social Issues in America: An Encyclopedia by