Le Morte D'Arthur

Le Morte Darthur is a 15th century Arthurian prose tale written by a knight named Thomas Malory. And Excalibur is loosely based on it, yes, although it differs in some aspects (the biggest difference, to me, is that Excalibur makes Arthur into a kind of Fisher King, while they are completely distinct characters in the book, as in most other Arthurian tales). Malory's work is quite unique in the English language, in that it is the only one that tells the legend from the beginning to the end of Arhturian times - prior to that, there are "episodes" of Arthurian legend here and there. It draws heavily on the earlier French Vulgate Cycle, which also tells the story form beginning to end (and some more, since it begins actually several centuries before Arthur, with the legend of the origins of the Grail and how it came to England). It also incorporates a lot of English and Welsh material. To many people, it is THE Arthurian legend, because it is so famous and so complete. Most modern pieces of Arthurian ficiton claim Malory as their source: Marion Zimmer Bradley's Mists of Avalon for example, or the Starz series Camelot... even when they hardly have anything to do with that book! Its structure also makes it one that most closely ressembles a modern novel, so it's not as hard to get into as some of the other medieval texts. But the language and style are still "archaic" to modern eyes and it can be a difficult read (especially as it is over 1000 pages long in its paperback edition).

You can get an idea of what it looks like by reading the beginning online here:
http://etext.virginia.edu/toc/modeng/public/Mal1Mor.html

This is an edition with modernised spelling, although it is the original 15th century text (there are two editions of The Morte Darthur, should you want to buy it: one is called the Caxton edition, as in the link above; the other is the Vinaver edition, in which the spelling is not modernized, but which has the preference of scholars because it is based on a manuscript, not a printed version; there are a few differences here and there between the two editions, but nothing dramatic).

"Occasionally I'm callous and strange."