This series is getting better. Part of it could be that Priest doesn't make this a sequel to Boneshaker so much as it's another story set in the same world. The main character is the daughter to one of the secondary characters in the first book, but otherwise this is a story about her, and about war, and about her journey across a divided country.
The other reason the series is getting better is because Priest took the time to develop her main characters this time around. In Boneshaker, Briar and Zeke were important, but so crudely drawn that they came across as caricatures that characters. With Mercy Lynch, Priest created a more realized character, someone you could relate to much more easily than those in Boneshaker. Given that Clementine also showed an improvement in character development, I wasn't surprised (though it does raise the bar for the next four books in the series).
Priest's other talent is in telling a good, rip-roaring tale. Lots of stuff happens in this novel, from start to finish, and the pacing was just right. Nothing was resolved too quickly or too easily, but neither did it take too long for those scenes. For the Sad Puppies who bemoan the lack of adventure tales in science fiction, they would serve themselves well to read this series.
My one complaint was that there was a disconnect between Mercy's narrative and her dialogue. She's portrayed as being uneducated, but smart, and in the narrative (nearly all of which is told from her point of view), it's clear that she's very intelligent. Most of her dialogue portrays that, as well, but there are times when she starts talking like a country bumpkin, which was a bit jarring. Her voice didn't fit her in those moments, and whenever I would stumble across them (which I'm guessing was to further portray her as a Southerner), I would think that someone else was talking.
It's possible to read this book without having read Boneshaker and still get all you need to know about the story and the world in which it exists, but I wouldn't recommend it. Boneshaker still reads quickly, and sets the stage well for this one. I'm eager to see what the next book in the series will bring.