Sword of the Rightful King by Jane Yolen
Sword of the Rightful King is an Arthurian retelling that doesn’t so much retell as reinvent in a quasi-realistic way. Kind of like King Arthur if it were a lot better and still had some magic.
Set in post-Roman Britain, this starts after Arthur has already been king for several years, but has not yet been fully accepted. Among his chief opponents is Morguase, known as the “North Witch,” who sends her sons to Arthur’s court in Cadbury, using at least one as a spy. A lot of the plot revolves around Merlin concocting a plot to use the “Sword in the Stone” gimmick after Arthur is king to give him more validity and increase his popularity, and certain things are left to the reader’s imagination as far as whether or not they were magic or cleverness.
The narrative switches up POV characters quite a bit, but I found the narrative shifts easy to follow, and the bulk of the POV is split between Gawain and Gawen, a boy who holds a grudge against Gawain, and who Merlin takes under his wing.
It’s not exactly a purist’s dream, but it has some pretty interesting takes on the various characters and standards of the legend, and it takes a few more obscure things combined with modern takes and runs with them. Also, I, uhm, may love it forever for its take on Guinevere.
Note: I’m writing this up without the book beside me. A few of the names are different and I couldn’t remember the exact spelling, so I went with a more standard one.