The book is based on an obscure murder ballad -- rather, on a modern reconstruction of it. (The period ballad, "The Famous Flower of Serving-Men", doesn't have much of a plot.) William Flower, a nicely-turned-out young man of no declared birth, stumbles out of a storm into the King's castle, declaring that he's looking for a job.
We then flip to the tower of a sorceress, Lady Margaret, who is arranging to have a household slaughtered by mercenary bandits. This turns out to be a flashback. (The book is plagued by poorly-marked flashbacks; I often had to chew through several pages before figuring out that somebody was the wrong age.) There is necromancy and kinslaying afoot, not to mention crossdressing, poorly-managed invasions, and the bitter disillusionment of teenage crushes.
The storyline adheres to Martin Carthy's recording strictly, in its main points -- except for the ending, which goes for realistic awkwardness rather than fairy-tale pat. I wish more of the storyline had done that. The ballad plot beats feel forced, and the "real" plot is a lot of reaction and secondary characters dancing around while waiting for the big ballad climax. The big climax is not, in fact, a genre-fantasy battle between William and Margaret -- I'm not saying it should be -- but maybe some kind of conflict would have served the novel? William barely does anything.
One could argue that this whole book is a slice-of-life portrayal of William in Cyngesbury, while in the background news reports roll in of Margaret slowly but surely shooting herself in the foot. Neither is particularly novelistic.
Positives: the life that we see sliced is realistically medieval, as far as I can tell. I'm no expert, but it felt like period idiom, period points of view, and period dirty jokes. Also, no cheap fantasy-cliche (or romance-cliche) conflicts driving the plot. Negatives: as noted above, not much driving the plot at all.
It's worth noting that Margaret, explicitly an Evil Witch (plots murder, spreads plague, summons demons from Hell, not a lot of ambiguity there), is not treated as evil in her half of the story. The narration follows her plaguing and murdering attempts with the same dispassionate focus as William's... well, William's attempts to run a kitchen and keep people from falling in love with him. (Really, that's his primary goal. You see why I keep squinting at the plot.) Anyhow, I was impressed by this narrative balance at first, but I eventually decided it wasn't good for anything; Margaret's foot winds up shot and that's the end. She is not even the heroine of her own story. Best I can say about it.