I really loved this book, but I haven't managed to write a review since finishing it more than two weeks ago - I wonder why not?
Let's started with what I liked. It's alternative history, which is a genre I enjoy - speculative fiction merged with history, what's not to like? And I like the scenario of this one, which is that the variant of smallpox that arrived in North America with the first Europeans was the non-lethal variety, so the Native American population was not devastated. And it ties really nicely into what I recall from The Dawn of Everything including how advanced Native American civilisations were, the existence of potential other routes of development, and Cahokia itself, a Native American urban civilisation.
Spufford has imagined how such a civilisation would have advanced into and alongside the modern, urban, industrial civilisation of the United States - the alt-history Cahokia is part of the US but still somewhat distinct politically and much more distinct culturally. It's well imagined and beautifully described.
I also like the way that Spufford has avoided those long passages explaining the course of the alternative history - in the best tradition of alt-history there's an author's after-note, but no long explanatory passages.
As in his book Golden Hill, Spufford - an English white bloke, as far as I know - has a great insight into the workings of race in America. The main character is of Native American descent but grew up in a white-run orphanage, so he both is and isn't connected to the Native American civilisation of Cahokia. And he's a Jazz musician, so his friends are mainly Black Americans, and he's partly connected to their world too.
The actual murder mystery plot, and the other characters - are great too. I was really sorry when it ended.
I'd like to say a bit more about the ending, which has some parallels with what happened at the end of The Lincoln Highway, but that would be a spoiler.