When writing an essay on a work of literature, you are engaging in literary criticism. Before writing your essay, you may find it helpful to know what previous critics have said about the text you are analyzing: existing critical interpretations can be found in both article and book form: check the links for Articles, Books, and History of Criticism.
Literary criticism does not exist in a vacuum. Every critical analysis of a text depends, whether implicitly or explicitly, on a theory—a theory of language, of history, of genre, or of subjectivity, for example. For identifying and developing your own theoretical approach, or for identifying and understanding the theoretical approach of articles and books you read, it can be useful to know something about literary theory in a broader sense.
The Johns Hopkins Guide to Literary Theory & Criticism
REF Z6514.C97 J64 1994
Online (2012)
There are many additional reference works (i.e. dictionaries, encyclopedias, histories) dealing with literary theory in the Reference section of the Library between PN44 and PN86. Here are a few particularly useful titles:
A Glossary of Contemporary Literary Theory
REF PN44.5.H37 2000
The Columbia Dictionary of Modern Literary and Cultural Criticism
REF PN81.C656 1995t
Encyclopedia of Contemporary Literary Theory: Approaches, Scholars, Terms
REF PN81.E54 1993t
The Cambridge History of Literary Criticism
REF PN86.C27 1989
Because "theory" is a very broad term, of relevance to students in many disciplines besides Literature, you will also find a number of useful books in other parts of the Reference collection, e.g. A Dictionary of Cultural and Critical Theory (REF HM101.D527 1996).
Finally, keep in mind that every call number in the Reference collection corresponds to a call number in the stacks. Thus, it is well worth going up to the fourth floor and browsing through the PN 44 to PN 86 range, where you will find a great many books on literary theory. Some particularly important resources are:
Lentricchia, Frank & Thomas McLaughlin., eds. Critical Terms for Literary Study.
PN81.C84 1995.
Barry, Peter. Beginning Theory: An Introduction to Literary and Cultural Theory.
PN81.B367 2002
Bressler, Charles E. Literary criticism : an introduction to theory and practice.
PN81.B666 1999
Voice of the Shuttle - Literary Theory
Academic Info - Comparative Literature: Literary Theory
Introductory Guide to Critical Theory (Dino Felluga, Purdue University)
Includes more than 240 entries on critics and theorists, critical schools and movements, and the critical and theoretical innovations of specific countries and historical periods.