Subject encyclopedias and dictionaries are useful for learning a little about a topic, seeing how it might be broken down or enlarged, and for finding further reading suggestions.
Oxford Reference Online contains many reference works from Oxford University Press, some of which are listed individually, below. At top right try entering: legend of the holy grail. Under that box, where it says "Search within my subject specializations," click on the drop down arrow and check the box for literature. ORO covers all subject areas. Those for history, literature, media studies, religion, social science, society and culture might be most helpful.
HISTORY ENCYCLOPEDIAS AND DICTIONARIES (Alphabetical by title)
Arthurian Figures of History and Legend, a Biographical Dictionary Main DA 152.5 R458 2011
Dictionary of Medieval Knighthood and Chivalry Main CR 4505 .B76 1986 and 1988 2 vols.
One vol. covers concepts and terms. The other covers people, places, events. Topical list of entries and bibliography in the back of each volume.
Handbook of Medieval Culture Reference CB 351 .H227 2015 v. 1-3
Link above goes to v. 1. Link to v. 2. Link to v. 3. Topical chapters, such as chivalry and knighthood, with selective bibliography at end of each chapter. Long bibliography at end of v. 3.
Historical Dictionary of Late Medieval England, 1272-1485 Main DA 175 .H56 2002
Medieval England, an Encyclopedia Reference DA 129 .M43 1998
Medieval France, an Encyclopedia Reference DC 33.2 .M44 1995
New Arthurian Encyclopedia Main DA 152.5 .A7 N48 1996
New Dictionary of the History of Ideas Main CB 9 .N49 2005 v. 1-6
Detailed index in back of v. 6. Long article on myth in v. 4. Extensive reader's guide at beginning explains layout of the set and its organization with geographical, chronological, and broad disciplinary approaches; use this to get a handle on it.
Oxford Dictionary of the Middle Ages Main D 117 .O92 2010 v. 1-4
Intended to succeed Strayer's Dictionary of the Middle Ages (Main D 117 .D5 1982 v. 1-13), but is not so extensive. Index in back of v. 4. Signed articles, with bibliographic references, less than a page to a few pages.
Oxford Guide to Arthurian Literature and Legend Main DA 152.5 .A7 L87 2005
LITERATURE ENCYCLOPEDIAS AND DICTIONARIES (Alphabetical by title)
Cambridge Companion to the Arthurian Legend Main PR 328 .C36 2009
Cambridge History of Medieval English Literature Main PR 255 .C35 1999
Companion to Arthurian and Celtic Myths and Legends Remote Storage PN 685 .D59 2004
Companion to Medieval English Literature and Culture, c1350-1500 Main PR 255 .C653 2007
Encyclopedia of Medieval Literature in Britain Main PR 260 .E53 2017 v. 1-4
Over 600 authoritative entries on key figures, texts, critical debates, methodologies, cultural and historical contexts, related terminology, and influences in the literatures of Britain from the fifth to the sixteenth centuries. Represents all the literatures of the British Isles including Old and Middle English, Early Scots, Anglo-Norman, the Norse, Latin and French of Britain, and the Celtic Literatures of Wales, Ireland, Scotland and Cornwall. Covers the material remains of Medieval British literature, including manuscripts and early prints, literary sites and contexts of production, performance and reception as well as highlighting narrative transformations and intertextual links during the period.
New History of French Literature Main PQ 119 .N48 1994
New Oxford Companion to Literature in French Reference PQ 14 .N48 1995
Oxford Companion to English Literature Main PR 19 .O94 2006. Online version is 2009.
Oxford Encyclopedia of British Literature Main PR 19 .O95 2006 v. 1-5
RELIGION AND MYTHOLOGY ENCYCLOPEDIAS AND DICTIONARIES (Alphabetical by title)
Dictionary of Celtic Mythology Main BL 900 .M445 1998 Online is 2004 ed.
New Catholic Encyclopedia Main BX 841 .C25 2003 v. 1-15
FILM ENCYCLOPEDIAS AND DICTIONARIES (Alphabetical by title)
Arthurian Legends on Film and Television Main PN 1995.9 .A75 O48 2000
Cinema Arthuriana Main PN 1995.9 .A75 C5 2002
Grail, the Quest and the World of Arthur
King Arthur on Film Remote Storage PN 1995.9 .A75 .K56 1999
Use of Arthurian Legend in Hollywood Film Main PN 1995.9 .A75 U44 1996