I didn't pay a lot of attention during US History classes, but I'm pretty sure the teacher didn't tell us about Cahokia; for me, my awareness of this onetime great Native American city had to wait until I read Before the Revolution, a fine history book by Daniel K. Richter. Cahokia was located across the Mississippi from where St. Louis is today; at its 12th-century peak,it might have had more residents than either London or Paris at the same time. It was abandoned pre-1491, and today it's just mounds and archeological digs.
Also: I read Francis Spufford's previous novel, Light Perpetual, and liked it fine. When I saw a WSJ review of this book, it was a must-get.
It's a wonderful combination of speculative alternate history, mystery, and thriller. Spufford doesn't reveal the cause of his novel's alternative timeline until the end, so I won't either. It's set in 1922, and a map at the front of the book reveals a significantly different USA, with a significantly stronger Native presence.
The hero, Joe Barrow, is a detective with the Cahokia police; he is summoned with his partner, Finn Drummond, to a gory scene: atop one of the downtown skyscrapers a corpse has been found on a skylight, throat slashed, and—ick!—chest opened, heart removed. Obsidian flakes in the cavity! How very Aztec! Or is that what we're supposed to think?
It turns out that the murder has much to do with Cahokia's power structure, superstition, the Ku Klux Klan (and other racists), and much more. Barrow (it turns out) is a talented jazz pianist, and his old bandmates really want him to quit his cop job and get back with the group. In addition to this conflict, Barrow undergoes a lot of torment: physical, mental, and (even) romantic.
My report on Light Perpetual called Spufford's prose "beautifully ornate" at times, and that continues here. But there are also a number of pulse-pounding action scenes. There are also some sly nods to figures from our timeline; an archived report of a disastrous US Army mission against the Natives is written by Captain Robert E. Lee. Barrow's investigation takes him across the Mississippi River at one point, landing him in St. Louis; in this timeline, it's a dinky town, merely the proverbial wide-spot-in-the-road.