An excellent but nasty book.
This is Abercrombie's best work to date. The tighter format of a single-volume story seems to help him, keeping this a little more focussed than the First Law trilogy, and his skills as a writer have definitely improved. The story also benefits from a reduced desire to make points about fantasy clichés, which I felt harmed the story and characterization in The First Law at times.
On the surface, the story is a simple tale of revenge. Female mercenary captain Monzarro Murcatto ("Monza") and her brother Benna are killed by their employer, Duke Orzo, who fears that her popularity might result in political ambitions. Except ... Monza isn't quite dead when they throw her off the battlements. Found by a mysterious stranger who nurses her back to health, she swears revenge on the seven men who were present; the Duke, his two sons, his bodyguard, his top general, Monza's traitorous second-in-command, and the banker who funds the Duke's wars.
Revenge, of course, is not as easy as it seems. It has consequences. You don't easily or lightly kill the powerful, and even a woman as cold-hearted as Monza, the Butcher of Caprile, finds less joy in it than she might have expected.
The setting is largely based on the warring city-states of Renaissance Italy, a fitting backdrop for Abercrombie's cynical straight-shooting about politics, war and power.
What makes this book for me, though, are the characters. Well-drawn, and despite the subject matter, much easier to sympathize with than many in The First Law. Also, there's a crucial difference; The First Law is largely about how things don't change, and most of the characters there do not grow. Here, the focus is on change; the impermanence of anything, perhaps, but still, the setting is not one for stasis and neither is the task, and the characters do change and grow, some for the better, some for the worse.
I wouldn't say that you absolutely need to have read The First Law; while some characters, not including Monza, do carry over from the previous work, none of the main characters in this played huge roles in what came before. Knowledge of the previous series certainly gives a bonus when we recognize things, but I think the story works without it.
If you can tolerate much blood and human nastiness in the pursuit of a story less hopeless than it may appear, I'd recommend this very highly indeed.