Background sources provide general information about your topic such as definitions, key terms or persons, and suggestions for more sources. Sources are often books such as dictionaries, encyclopedias, handbooks, although they may be websites, articles, and similar tools. A database with some good background sources is CREDO Reference.
CREDO is listed in the Database list. 
Examples of background sources (Encyclopedias and More) in Boreham:
Black's Law Dict. Crime and Justice Criminology Law Enforcement Police Science Social History of Crime and Punishment in America