I've been confused about the legend and books of King Arthur. It seems to me that a number of authors wrote books over a several hundred year period. Could anyone who wanted to just pen a book about the gang? How was established canon decided on. Is it possible to compare this to say, the Star Wars expanded universe of books?
I'm thinking specifically of the idea that Lancelot was having an affair with Guenevere (Spoilers?), Wikipedia says this was invented in le Chevalier de la Charrette, did other writers just decide to pick up on this? Finally when the books were written was it the authors making up new stuff and writing it down, or did the legends already exist as part of the culture?
Think of it as fan fiction unleashed.
Yes, anyone could write on The Matter of Britain (Arthurian stuff), or The Matter of France (stories of Charlemagne and his paladins), or The Matter of Rome (Classical epics). Or all three, as when in Orlando Inamorato they had the paladins and a love potion fountain left by Merlin and the sword of Hector!
Frankly, there is no canon. Anyone could change it as they like. It began with folk tales and songs about a warrior king who may or may not be based in some level of history. These became popular and expanded. Then writers, people whoactually wrote stuff down, started recording stories. At the very least, they redacted and tidied things up.
All this time, society changed and the stories reflect their times, not the far past when this supposedly happened. Courtly love became a thing, and no queen not a pious dog would not have a gallant lover, and true love between husband and wife was not possible, so Chretian de Troyes invented Lancelot out of the whole cloth, began the process of making King Arthur an old has-been rather than the central hero, and then invented Galahad to have a pure and non-courtly pious hero for his entirely invented Grail Quest.
Chretian died with his Grail story unfinished, but his other stuff had been so popular it was published anyway. At least two other authors "finished" the story in their own way, so there is no one canonical Grail story dating back to ancient days. Then we go into a period that sees lots of prose Arthuriana being published.
Compare , not only the style, but the cast of the Welsh stories, like Kilwich and Olwyn with stuff based on Lancelot and Galahad.
On the eve of the Renaissance, Sir Thomas Mallory collects and redacts these stories to his own satisfaction, and that of innumerable readers. One of the earliest printed books was his Morte d'Arthur because it was a sure seller. This also made it the most widely distributed Arthuriana, so that many treated this as canon. But most writers still did whatever fantasy story they liked with the material.
The same thing happened with The Matter of France. First we got Orlando in Love by one author, never finished, and then another took up the story in Orlando Furioso. There were also original fantasy heroes like Amadis de Gaul (which actually means of Wales).
So this is the source of the whole Northern Thing high fantasy story, in the Matters of Britain and France. They began as folktales, were created by many hands over perhaps a thousand years, occasionally redacted, had no controlling authority.
As well, copyright as we know it was pretty non-existent until the early 20th century. If you wrote something popular, anyone could try to use your characters. For the most graphic example, War of the Worlds by H. G. Wells was a tremendous success in 1897. Though it appeared in the US in Cosmopolitan magazine, a heavily rewritten version was serialized in the NewYork Evening Journal as Fighters of Mars. Then in 1898, the Journal paid science writer Garrett P. Serviss to write a sequel to WotW, which gave the world Edison's Conquest of Mars (which originates a lot of scifi tropes but if a farrago of fervid nonsense in a lot of places). No one included either Wells or Edison in this project. No one sued about it, either.