This is about two things: slavery and living in the countryside. And lots of other things too, probably. Riverboats, possibly.
I saw someone somewhere dismiss this as another “some new species stands in for a race of humans” story, which is unfair. OK, yes, in this case goblins are being transported to tobacco farms on the far side of the circle sea and it’s clearly oh no, slavery. But it isn’t just about that. It’s about the class of people who treat other people (who are different from them) as things AND how they make those other people feel like they deserve to be treated as things. And that’s the real point of the book, I think, that you should be aware of how the way you treat people makes them feel about themselves.
I know it’s not a very sophisticated moral point, but morals aren’t really all that complicated when you get down to it.
There’s some really cool stuff with Sam on the riverboat and I liked the descriptions of the Ramkin village. Seems like Pratchett has been spending time around where I grew up – the settings for this and the Tiffany Aching books are very reminiscent of home. I could be entirely wrong, of course.