There are several reasons to consult reference books. Specialized dictionaries and encyclopedias can suggest ideas for research projects as well as provide background information. Articles will often include suggestions for further reading and cross references to related subjects. Book-length bibliographies present important books and articles selected by an expert in the field.
American Historical Association's Guide to Historical Literature by 
Encyclopedia of American Economic History:Studies of the Principal Movements and Ideas by 
Encyclopedia of American Social History by 
Encyclopedia of the United States in the Twentieth Century. 4 vol. by 
Encyclopedia of Urban America by 