Half the World is the second book in the Shattered Sea series by Joe Abercrombie. It follows Half a King (which I discussed here).
This second book doesn't treat its protagonists quite as badly as the first book did Yarvi. They're not enslaved, or betrayed by family, and both are sound of body - though they do get plenty of injuries along the way. There's still plenty of troubles for them, though. Almost everyone in the story is caught up in the intrigue and manipulation that Yarvi engages in while trying to win allies to keep their home safe, none more so than Thorn. And both Brand and Thorn contribute to their own unhappiness when they fall for one another, but fail to communicate, such that neither believes the other returns the feelings.
Several of the characters from the first novel make an appearance, and there have been changes. Yarvi in particular is much more self-assured, but also colder and more ruthless. He's no longer the viewpoint character, but his actions still drive the larger story-line that sweeps people like Thorn and Brand along in its wake.
I don't believe it's really necessary to have read the first book to enjoy Half the World. I certainly recommend it, since there are many references and actions by various characters that make more sense when you know the prior story, but it would be possible to start with this book. That's a tribute to Abercrombie's writing, which is clear on explaining what is necessary to understand the current book, while at the same time referring to earlier events.
Much more of the world of the Shattered Sea is revealed in Half the World, as the travelers journey to foreign lands. They go as far as the "First of Cities" where an Empress reigns who is more wealthy and powerful than all the kingdoms of their home sea. More references are made to the ancient world, including a priceless still-functioning artifact meant as a present for that Empress. It seems pretty clear that this world came about after some kind of apocalypse ended a technologically-advanced civilization (possibly our own). The details of that history aren't important to the story, though, which is good since time has caused those details to be lost to the characters.
Half the World wraps up the stories of Thorn and Brand nicely, while still leaving open the fate of the kingdom (largely Yarvi's responsibility). The setup for the third novel is present, but I didn't feel like that was all that this book had to offer. Half the World is a fine story in its own right, not just the middle of a larger story-line.