When you're starting research, getting a broad view of related topics can be helpful. The resources below are just some that might be useful for research in English literature.
Brewer's Dictionary of Phrase and Fable
The Cambridge Companion to Modernist Poetry
The Cambridge History of American Literature, 1590-1820
The Oxford Companion to African American Literature
The Oxford Handbook of Medieval Literature in English
The Oxford Handbook of Science Fiction
The Oxford Handbook of Shakespeare