Caveat: This is the third book in a trilogy, and I haven't read the first two. However, I personally think that books in a series should be able to stand on their own.
The concept...King Arthur retold as urban fantasy in a gang-war setting with mainly African-American and Latino characters...is awesome. The execution is somewhat lacking. It starts slow and even towards the end the pace is glacial, especially for a story with this one's violence quotient. Some of the Arthurian and fantasy elements...notably the Grail quest and the existence of Fae...seem a bit tacked on, as if the writer really wanted to tell a story about coming up hard, families, and betrayal and didn't weave the mythic elements in deeply enough. They don't seem integral; you could strip them out and it would be essentially the same story. I still don't know how magic is supposed to work in the world of the story or what its deeper significance is.
Some of the problems I would describe as technical. The eARC needs another editing pass (a seven-year-old is suddenly twelve only a few pages later, and a character description is repeated verbatim twice in different chapters). However, some issues are created by the specific choices the author made; the story is written in universal 3rd person, and there are many POV shifts and what you might call expositional telepathy....that is, we listen in on the character's thoughts as he or she just happens to be musing on things the reader needs to know. This happens a lot. The effect is to slow down the pace and undermine the immersive quality of the story. The prose also suffers from what I have come to think of as the Edible Person/Descriptive Default problem: that is, the coloring and complexion of all of the PoC's in the book are described precisely, often with food adjectives ("coffee," "mocha," "toffee," "honey"), while all the white people are just white.* The prose style smooths out in the later portions of the book and the pace moves more briskly...but this is the third installment in a trilogy, not the first.
I liked the Guenevere character ("Lady G") and also liked the fact that she didn't just disappear into some nunnery-analog, hand to forehead; however, she doesn't seem to do a whole lot other than skulk about feeling guilty. There are several more active female characters, but interestingly Lady G is the only one who seems to have the aura of the mythic about her that the story needs in order to evoke the source material; even Nine (ie, Nimue) is a bit prosaic and her motives, though they may have been explained in one of the earlier books, are unclear in this one.
I might be a hard sell here; I read The Once and Future King when I was twelve and loved it to pieces. I've also read Idylls of the King, A Connecticut Yankee in King Arthur's Court, The Mists of Avalon...you get the picture. And yet, when you take on a story like this, you choose exactly that kind of difficult challenge. Is it fair to complain that a book could have been brilliant but isn't? There's some potentially good stuff in it...social commentary and philosophical underpinnings...but it just doesn't quite gel. I like the idea of the book better than I liked the actual book itself. Arthur is such an old story with so many re-tellings that it's hard to bring a new twist to it; in that much, King's War succeeds (and, I assume, the preceding two books as well). If you find the subject and setting compelling and are not as easily distracted by prose tics as I am, this book might work for you. It didn't, so much, for me.
*I grant this is the kind of thing that many readers wouldn't notice, but the fact that it's invisible is kind of the problem. Anyhow, K. Tempest Bradford was on a WisCon panel about describing non-white characters and made a crack to the effect that if she were really chocolate she'd be tempted to chew her own arm off. And now I can't unsee it. Thank you, Tempest.