Alan Lupack's "Post-Medieval Arthurian Literature in English (Other than Fiction): A Preliminary Bibliography"
this took a lot of effort and I'm incredibly jealous of what he has produced; it's a better specific bibliography than anything I will ever produce on this website

Internet Medieval Sourcebook, edited by Paul Halsall
just lovely

The End of Roman Britain by William Bakken
invaluable

Encyclopedia of the Celts, compiled and edited by Knud Mariboe
a great reference, built as freeware