Sources of Information for Historical Research by 
Reference Sources in History by 
United States History by Credo Reference provides an online reference collection of over 600+ reference books. This collection includes a variety of dictionaries and encyclopedias, as well as a broad range of subject-specific titles covering everything from art to geography and law to technology. See, for example, this overview on the East African theater and the role of the Carrier Corps and its successors from The Encyclopedia of African History within CREDO: KENYA-World War I-Carrier Corps.
American National Biography offers portraits of men & women from all eras and walks of life whose lives have shaped the nation. Published in 24 volumes in 1999, the ANB essays, created by scholars chosen for their topical expertise, are updated semi-annually online, with hundreds of new entries each year and revisions of previously published entries to enhance their accuracy and currency. The ANB Online features thousands of illustrations, hyperlinked cross-references, links to select web sites, and powerful search capabilities.
The Oxford DNB is a collection of over 56,000 specially written biographies of men and women who have shaped all aspects of the British past. Updated three time per year with new material published every January, May and October. LIMITED TO 1 USER AT A TIME.
Both these biographical sources present the historian with two very powerful tools: extensive bibliographies of primary and secondary sources appended to the entry of each biographee and the ability to search the full text of every entry for particular events, organizations, and places and thereby bring together information about numbers of people associated with those places and things.
The editors of The Chicago Manual of Style provide a website to support use of their style guide. Although the actual text of the manual online can be seen only by subscribers, a Chicago-Style Citation Quick Guide on constructing some of the most often used types of citations is provided for free.
Purdue University's Online Writing Lab [OWL] makes a more detailed guide to Chicago style available on their website.
Since the Rhode Island College History Department uses the Chicago bibliographic citation style in a modified way, the final arbiter of correct usage is your professor.
The Chicago Manual of Style was not created to support the needs of historians specifically. And so, though it is hundreds of pages long, there are still some unusual items, especially primary sources, which Chicago may not address thoroughly or at all. Consider consulting Evidence explained : citing history sources from artifacts to cyberspace by Elizabeth Shown Mills in the Adams Library Reference Collection: Ref D5 .M55 2007 for examples of sources such as cemetery records, voter rolls, deeds, etc.