There seems to be many conflicting stories which have been released over the years about King Arthur. Countless sagas and novels have been written either directly about him or about his kingdom of Camelot.
It's best to keep in mind that there is a whole premise about Arthur that, one, he never existed and is just a legend that is used often because he is so familiar a name to most people, or two, that he was a real person who drew the world out of the Middle Ages and influenced most of ancient Europe.
Usually, Arthur is associated as being the King of England and ruled from his court at Camelot. There, he had a round table which placed all his knights equal to him and each other so that no one man had more of a say than any other. Arthur had a personal advisor and friend named Merlin, whom was also a magician. Merlin raised Arthur to be king and to rule justly.
Lancelot was Arthur's finest knight. He was humble, courageous, chivalrous and the ideal warrior. Unfortunately, Lancelot fell in love with Guinevere (Arthur's wife) and they had an affair. Arthur meanwhile was magicked by his sister Morgause and they had an incestuous baby together named Mordred. Mordred was the antagonist of Lancelot and Arthur and they would end up having a battle.
Not all stories follow this premise and some follow none of it! But this is basically the widely held cycle of Arthur.
The first classic novel written about Arthur and his knights was
Le Morte D'Arthur by Thomas Mal(l)ory, sometime in the 1400's I believe. Personally, I haven't read it but I've heard it is quite excellent if you enjoy reading dated material.
More recently, there has been plenty written on the topic. I read T.H. White's adaptation,
The Once & Future King, which is an excellent novel. Most people are familiar with it's introductory story
The Sword In the Stone which was produced into that famous 60's movie by Disney. I will say though that T.H. White writes in more of a philosophical sense than most and quite a few people find his novel hard to digest.
“The destiny of Man is to unite, not to divide. If you keep on dividing you end up as a collection of monkeys throwing nuts at each other out of separate trees.”
- T.H. White, The Once & Future King
Two other epics have been written related to King Arthur's world. First is the Avalon series by Marion Zimmer Bradley. I've never read any of these books yet they seem to get rave reviews. Second is the epic by Jack Whyte. His series is supposed to be obsessive in detail and also be the most comprehensive series ever written about Arthur. So much so in fact that I believe that there is over a half-dozen novels on the subject. I own the first story,
The Skystone, but have yet to read it.
There are undoubtably many other novels on the topic of Arthur, Lancelot, Merlin and company and I hope some other people mention them and their opinions.
Hope this helps a bit.