Start with high-confidence literary criticism reference books, then narrow by subgenre or adjacent topics.
The Oxford Guide to Arthurian Literature and Legend
Alan Lupack
Notes on Footnotes
Melvyn New, Anthony W. Lee
Notes on Blood Meridian
John Sepich
Victorian Culture and Society
Adam Charles Roberts
The Norton Introduction to Literature, With 2016 MLA Update, Shorter Twelfth Edition, Paperback Version
Kelly J. Mays
James Joyce's Ulysses
Stuart Gilbert
Ex Libris
Michiko Kakutani
The Complete Christie
Matthew Bunson
Old English Literature
Fred Robinson
Merriam-webster's Encyclopedia of Literature
Merriam-Webster, Photos
American Lit 101
Brianne Keith
Stephen King's the Dark Tower Concordance
Robin Furth, Stephen King
Beginning Theory
Peter Barry
Thrillers: 100 Must-reads
Hank Wagner
Tale of Genji: a Reader's Guide
William J. Puette
