While research in the hard sciences and the social sciences tends to appear more often in articles, research in the humanities is frequently a matter of books. A great deal of research carried out in the field of American History appears in books, technically called "Scholarly Monographs." Therefore, a good place to begin your search, once you have narrowed your topic, is the online catalog.
United States History--Eighteenth Century
The historical period will be followed by increasingly specific subjects headings, such as "United States History-- 19th Century Caricatures And Cartoons"
You can begin to browse the subject headings for American History by visiting this catalog page