However, here are the main ones:
There are many compilations and translations of the stories of Arthur, from Mallory’s Morte d’Arthur onwards. All are fairly good and all have something new and different to offer.
The Mabinogian looks at Arthurian stories from a different angle.
In terms of contemporary fiction, Marion Zimmer Bradley’s Mists of Avalon is excellent, as is Stephen Lawhead’s cycle of books. There are others which I have read which give a new slant on the mythos, but I can’t remember exact details. Take it all with an pinch of salt and mix it all together to give ideas.
Scattered through Michael Moorcock's works are references to the Grail Family and the Atlanteans and their descendants. All of which is fascinating stuff.
Any book on Celtic Myths will give an idea of the deities involved and how they wee seen. Irish myths include the Ulster Cycle but there are also other folk tales and myths. (I was reading a book of tales of the Irish Gods before I came to Moscow, but it is in Ireland.)
There are a whole load of books dealing with Roman Britain and the end of the Empire there. The Oxford University Press has a series of books on English History, the first three books cover Roman Britain, the English Settlements and Anglo Saxon England. Also, there is an excellent book called "The Age of Arthur", can’t remember who by, though, which covers the end of the Romans and the rise of the Saxons and looks for evidence of Arthur in a nice level manner, not one of these new age books where they firmly believe in Arthur and point to silly evidence of his existence.
Holy Blood, Holy Grail by Leigh et al. Is an interesting read, as are its successors The Messianic Legacy and The Temple and the Lodge. Many of their conclusions are suspect, but in a Fantasy Campaign, they can be easily worked and used.
Any book on the lives of the Saints is worth a read as it tells something of the Saints who were recognised at this time. They also give something of the mythical history of the areas which can be included in any campaign.
Other interesting reads, whilst not directly applicable, can ad to the background and can offer interesting ideas which can be incorporated into a campaign. These include books on Freemasonry, Gnostics, the Templars, Christian Apocryphal works, European Folk Tales etc. At one time, I had far too much time on my hands and could afford to read extensively.