A subject heading is a special type of category that groups together materials in the Library that share a topic, author, discipline, or particular format. While simple keyword searches are a good start for finding materials, a subject heading search is an advanced type of search that often produces much more precise and targeted results that are often exactly what you need.
The subject headings used in the Library catalog are standardized Library of Congress headings, which may be subdivided by geographic area, chronological period, genre, or sub-topic. The language of subject headings is not particularly intuitive or natural, so you shouldn’t hesitate to ask a librarian for help in finding the correct subject headings.
A good way to find catalog subject headings for your topic is to do a keyword search in the Library's Primo Catalog. For example, let's say you look for "Agatha Christie" and you see the following record in your results:

Look in the "Topics" field: these are the subject headings for this book. The subject headings in the online catalog are hyperlinked, so simply click on the heading you're interested in, and it will automatically launch a search for books under that heading.
It's best to use both fairly broad headings as well as specific ones, in order to make sure you don't miss any relevant material. You'll likely find multiple headings that are relevant to your topic, and you will find a variety of resources if you look at all of them.
Use the following subject headings to search for books, periodicals, and electronic resources in the Primo Library Catalog:
American literature--19th century
American literature--19th century--History and criticism
American fiction--19th century
American fiction--19th century--History and criticism
American poetry--19th century
American poetry--19th century--History and criticism
Realism in literature
Transcendentalism (New England)
Romanticism--United States
African American and Southern Literature
American literature--African American Authors--History and criticism
American literature--Southern States--History and criticism
Southern States in Literature
Women Writers
American literature--women authors--History and criticism
Women and literature--United States--History--19th Century
For African American literature, see the Literatures and Languages Library's guide for this subject: African American Literature Guide.
Uncle Tom's Cabin & American Culture-- a multimedia archive at the University of Virginia