The sources for my research are divisible into several categories.
Books on the history of Wales and the Dark Ages include:
A History of Wales, by John Davies.
Welsh Castles, by Adrian Pettifer.
Castles of the Welsh Princes, by Paul R. Davis.
In Search of the Dark Ages, by Michael Wood.
For King Arthur:
Four Ancient Books of Wales, by William F. Skene.
Pendragon and The Keys to Avalon by Steve Blake and Scott Lloyd.
King Arthur and His Knights: Selected Tales, by Thomas Malory and Eugene Vinaver.
The History of the Kings of Britain, by Geoffrey of Monmouth and Michael A. Faletra
King Arthur, by Norma Lorre Goodrich.
Legends of Blood: The Vampire in History and Myth by Wayne Bartlett and Flavia Idriceanu.
In Search of Dracula: The History of Dracula and Vampires by Raymond T. McNally and Radu Florescu.
And then the Fiction, of which this is only a small sample:
Twilight of Avalon, by Anna Elliott
Mists of Avalon, by Marion Zimmer Bradley
Avalon, by Stephen Lawhead
The Arthurian Saga, by Mary Stewart
The Sword in the Stone, by T.H. White
A Connecticut Yankee in King Arthur’s Court, by Mark Twain
The Arthur Books, by Bernard Cornwall
The Forever King Trilogy, by Molly Cochran and Warren Murphy
Firelord, by Parke Godwin
Sword at Sunset, by Rosemary Sutcliffe
The Kitchen Knight, by Margaret Hodges
Idylls of the King, by Lord Alfred Tennyson
Sunshine, by Robin McKinley
Dracula, Bram Stoker
The Historian, Elizabeth Kostova
The web sites I use are truly too innumerable to recount here, but include:
The Celtic Literature Collective, http://www.maryjones.us/ctexts/index_welsh.html
Welsh Castles, http://www.castlewales.com/
Early British Kingdoms, http://www.earlybritishkingdoms.com/
The Camelot Project, http://www.lib.rochester.edu/camelot/
The Ruin and Conquest of Britain: http://www.cit.gu.edu.au/~s285238/DECB/DECBps.html