So, there's been a lot of these. Le Morte D'Arthur is probably the most influential. My personal favorites are the Warlord Chronicles by Bernard Cornwell. Those books are some of my all-time favorites. The Once and Future King (which I'm currently re-reading) I also found really good. I haven't yet read the Mary Zimmerman Bradley stories though its been on my list to read for decades. I think the whole legend is really poetic and classic. It has to be really hard to write a unique Arthurian novel in basically the oldest fan fiction community that has ever existed. Any thoughts on novels about King Arthur or the legends?

Btw I always thought Gawain and the Green Knight was a very underrated story.