Nancy Springer's I Am Mordred and I Am Morgan Le Fay have been popular for years with my fans of darker literature. Since I loved her Enola Holmes and Rowan Hood series, I figured I would not be disappointed, and I was right. It is always interesting to hear a well-known story told from another point of view.
A similar title, if you can find one languishing on the shelf, is Rosemary Sutcliffe's The Road to Camlann(1982), but Mordred is not portrayed as sympathetically. Her The Sword and the Circle is a more standard version.
There has been a lot of interest in Arthurian legend this year. Probably the most popular series has been Gerald Morris' The Squire's Tale (8 books), which has a Monty Pythonesque flavor to it.
Crossley-Holland's At the Crossing Places has additional layer of fantasy, involving time travel between two historical periods that is appealing.
McCaffrey's Black Horses Before the King has a lot of adventure and war.
To follow Merline, Barron's The Lost Years of Merlin series and Yolen's Hobby, Passager, and Merlin are good.
Seems like there should be more of these titles. Maybe I read them at other libraries.
Best overall-- Yolen's Sword of the Rightful King. No longer remember details, but the ending surprised me and I loved it.