Reference sources, such as dictionaries and encyclopedias, will be useful for refining your topic and coming up with keywords. Some of entries will contain bibliographies which you can use to locate key books and articles on your topic.
The Cambridge History of American Literature, 1590-1820 by 
The Cambridge History of the American Novel by 
The Cambridge History of American Women's Literature by 
The Cambridge History of African American Literature by 
Historical Atlas of the United States by 
The Oxford Companion to American Military History by 
Atlas of American Military History by 