Reference resources are a great place to begin your research. They can help you define terms as well as to find basic information about a topic, a chronicle of its history, theories, key people and sometimes a bibliography of additional sources.
There are many different types of reference sources, which include dictionaries, encyclopedias, thesauri, bibliographies, handbooks, maps, style manuals to name a few.
Handbooks and companions provide a survey of the state of research about a topic or region, historiography or concise overviews as well as ongoing debates among historians.
A Companion to Modernist Literature and Culture
A Companion to Modernist Poetry
A Companion to the Modern American Novel, 1900 - 1950
The Cambridge Companion to Modernist Women Writers
Handbooks, Companions about Specific Modernist Authors
Cambridge Companion to T.S. Eliot
Cambridge Companion to James Joyce
Cambridge Companion to Virginia Woolf
Cambridge Companion to Thomas Hardy
The Library has several volumes in the series, The Edinburgh Companions to Literature
To find titles, do the following search in Omni.
The OED is the most comprehensive and authoritative dictionary of the English language.