Lackey has been writing Mary-Sues in fantasy crack-fic for decades now, so this book came as a surprise to me. She's clearly put in research into early Celtic life and tales--her Gwenhwyfar serves mead and ale with her own hands in a great hall filled with dog shit. This is probably the best book she's ever written--certainly it's the most controlled. But still, sadly disappointing. If you tackle Arthurian legend after centuries of people messing with it, you'd better have something new and interesting to say. Lackey definitely doesn't, but it's not a complete failure as a book. The world felt possible, and neither the "Old Ways" nor the "White Christ" felt demonized (a common pitfall of the Arthurian tales). Most of the story is about Gwen becoming a warrior--she doesn't even meet Arthur until 3/4ths of the way into the novel. I'd ordinarily really enjoy this feminist twist on what makes Gwen important--that it's her skills, rather than her marriage--except that what Gwen is
involved in is pretty boring. Even the training montage doesn't have the same vitality as, say, in Arrows of the Queen. By controlling the worst of her Mary-Sue-creating tendencies, Lackey has removed a lot of the fun in her writing style.
Which is not to say that Lackey has learned to write reasonable characters. Her Gwen is perfect in a very well-worn way: she's beautiful, but doesn't pay attention to clothes or makeup! she's more gifted with magic than anyone else! she's the best rider and scout in the army! the fey do as she bids, and come when she calls! she still looks 16 in her late twenties! (Lackey emphasizes how preternaturally young Gwen looks, in nearly every chapter. Why is this important?) She's an unbelievable character, but less so than the Evil Characters. Why do people do mean things? Because they are Evil! From the very moment they are born, they are Evil. Constantly, irredeamably, in every possible situation, Evil. Arthur's kingdom falls not because the Saxons are invading, or because he didn't leave a good infrastructure in place, or because he was a bad king--but instead, because a character is Evil and so does all sorts of Evil things. (In case you weren't sure he was Evil, he rapes the main character repeatedly. I am so, so, so over rape-as-marker-of-evil in fantasy novels.) It's frustrating!
This is better than Lackey's usual fare, but isn't anything special. If you're looking for Arthurian legends with a strong female heroine, I highly recommend Jo Walton's The King's Peace instead.