There is a rather large difference between saying slavery was not the only factor in the Civil War (even your fictional steampunk Civil War), and ignoring it as a factor. But still, to have one of your protagonists be a fugitive slave having to go back into dangerous territory, and then, not ever really having it be an issue, other than that he doesn't get served at one bar? You can't have your cake and eat it too, and that's never been a metaphor that's made much sense, but I guess what I'm trying to say is that you can't have it both ways. And you can't ignore it. You introduce slavery, you can't just then handwave away having that mean anything.
This is too weighty a thought for a book this slight. But that's part of the problem. I get not wanting to delve entirely into slavery as the main topic of your steampunk fantasy - but you can't bring it up, and then ignore it.
But if I put that aside, what's left? Mostly fun, but not a whole lot more. Captain Croggan Hainey, former slave, former captain of the ship (I can't remember if it was formerly Confederate or Union) The Free Crow, had it stolen out from underneath him, and is now pursuing it across the Rockies into dangerous territory in Kentucky and environs. All he wants is his ship back.
Meanwhile, "Belle" Boyd, former famous Confederate spy, now put out to pasture, is working for the Pinkerton detective agency. As her very first mission, she's sent after the Free Crow, now named Clementine, to make sure it does get where it's going to go. But she discovers that what it's carrying might have some very real fallout for the side she spent her life serving, and her new loyalties are torn. (Might it have been more interesting if she'd been with the Pinkertons for a while, and it was a struggle between two loyalties?)
With nary an eye batted between them, Hainey and Boyd team up to
And really, I did enjoy this book. There was just this niggling "shouldn't the historical context that Priest is drawing on have some actual impact?" thought that kept running through my brain. If it hadn't been there, I would have been thoroughly amused by this romp. As it was, it was still quite fun. But not a lot more. Which is part of the problem. To truly be that light, you can't draw on something that heavy and then ignore it.