Half a King is the first book of a trilogy, but also works well as a standalone novel. There are plenty of ties for potential use in sequels, but no major threads left hanging.
This book doesn't work quite as well as The Blade Itself. This is a well-crafted new-style fantasy (gritty and realistic rather than epic and wishful); it offers a ready cast of likable characters, puts them in genuine danger, and yet never strays too far from a core of lawfulness and wish fulfillment. When it does (suggesting rapid changes in Yarvi's character), it wanders away from plausibility with a setup that might work in a larger book, or over the course of the trilogy, but the shift is a bit too sudden to be credible here.
Credibility is one of the weak points of the book. A pursuit forms a core element, but it simply seems far too unlikely to hold up. Instead, the author's intent shows rapidly through, weakening the drama that was the purpose of the pursuit mechanism. This is true more broadly - the risks and solutions posed are a touch too simplistic for the grim tone the narrative employs. The result is a mildly dissatisfying older-young-adult feel - there's danger and mayhem, but you have a feeling it will mostly work out in the end.
All that said, it's an enjoyable, competently written book, and a quick read. In some ways it reminded me of Harry Turtledove's Scepter of Mercy series (as Dan Chernenko).
Overall, recommended for Abercrombie fans, and those looking for solid fantasy adventure. Plus, because this book stands alone, it's easy to test the series out and see if you want more.