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.
Literature
English literature--20th century
English literature--20th century--History and criticism
English literature--20th century--Women authors
English literature--21st century
English literature--21st century--History and criticism
English literature--21st century--Women authors
Modernism (Literature)--Great Britain
Postmodernism (Literature)--Great Britain
Fiction
English fiction--20th century
English fiction--20th century--History and criticism
English fiction--21st century
English fiction--21st century--History and criticism
English fiction--Irish authors--20th century
English fiction--Irish authors--21st century
English fiction--Irish authors--History and criticism
Poetry
English poetry--20th century
English poetry--20th century--History and criticism
English poetry--21st century
English poetry--21st century--History and criticism
Drama
English drama--20th century
English drama--20th century--History and criticism
English drama--21st century
English drama--21st century--History and criticism
The Cambridge history of twentieth-century English literature
by
A concise companion to postwar British and Irish poetry
by