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
Can't find something you need? The interlibrary loan service can obtain the books or articles you are looking for.