In fairness, it's relation to the earlier work is fairly limited, with Best Served Cold being set in a different part of the world, several years later, and only the odd cameo from major players and larger roles for some bit-part-players to connect it at all. It's actually something I approve of; I'm not a huge fan of every fantasy world being the venue of one long epic tale (even though I like some of these epic tales) and it's nice to the world just being treated as a place to tell self-contained stories with a disciplined focus. I think you could read this without having read any of the earlier books and still get full value out of it.
So what we have is a blood-spattered revenge story, opening with the betrayal (and attempted murder of) mercenary Monza Murcatto, sending her off on a path to vengeance with a motley collection of unreliable allies. The book follows a loose structure of a new town, and new "job" for each target of Murcatto's list, to the backdrop of the long-running war she in the process of winning winding to it's conclusion. As with any revenge story, it gets darker and bloodier as it goes along, but does pull some interesting tricks along the way.
Chief of these is that traditionally, in revenge stories, you move up the list towards the badder, and more deserving targets of wrath, but here it's almost completely reversed. The further Murcatto goes, the less each victim seems to deserve it, and the more compromised and futile her quest for vengeance is. I'm seen Abercrombie accused of reveling in the darkness of his work, but Best Served Cold is if anything about the pointlessness and self-destruction Murcatto is inflicting on herself, and in it's best moments is uncomfortable not for glorifying this sort of carnage but for shining a light on where it takes you. I also want to applaud a pretty cool literary fake-out in the guise of a graphic sex scene too - felt totally gratuitous until it's reveal.
I've found anything to support this, but I suspect Abercrombie is a fan of noir writers like James Ellroy, and this is a novel that owes more the "Knights in Tarnished Armour" of that genre than it does to the fantasy epics it superficially resembles, and sits beside on the shelves. Best Served Cold uses it's secondary world to allow it's story to function as it should, rather than being a story being told to show off the world, and is all the better for it; a gripping, curiously thought-provoking read all the way to its (slightly too neatly wrapped up) end.