The premise is it is an alternate history version of Earth during the Wild West expansion. Rather than gold, the most valuable substance is the mysterious "crystal" that possesses many mystical properties that amount to allowing magic to be used. It is, understandably, sought by many-many prospectors. The town of Medicine Rocks, which is a name none of the settlers understand, is full of fortune hunters willing to do anything to get more of it.Despite the name, the book is actually about three separate women who share the narrative.
The first is the titular Mary Good Crow, a young half-Lakota woman raised by Catholic Nuns and subject to all the racism you would suspect. Mary is capable of "hearing" crystal and serves as a guide through the labyrinthine mines beneath Medicine Rocks. This despite the fact the prospectors are as inclined to cheat or assault her as pay her for her time. Mary is a really nice young woman but a bit too demure and polite, which may be a literal survival mechanism in her case. Those expecting a spitfire with her will be sadly disappointed but the other two more than make up for that.
In addition to Mary, there's also Josie the (seeming) rich girl from back East who is coming to revive her uncle's failing mining business. She wants to become a rich mining baroness and achieve independence but has found the majority of her inheritance has already been stolen by the crooked forces inhabiting this town. Finally, there's Ren, who has allied herself with those corrupt forces but is Josie's only friend. Ren would do anything to find out who killed her family and that includes betray a childhood companion who wants to go into business with her. She's also got haunted crystal pistols containing the ghost of her insane evil wizard father. That's a thing, apparently, yeah.
It is not a romanticized view of the West since much is made of the US cavalry's terrifying effect on Mary Good Crow as well as the fact they fully endorse Custer's cowardly attempt to take the Sioux nation's women and children hostage. That particular event is happening simultaneously with the book and threatens to upend the plot about locating a mother load of crystal. Some people will love the tie-in to actual history while others may feel its in poor taste. I am more of the former than the latter.
Oddly, in addition to the Wild West, the book strongly reminds me of Final Fantasy VII. The harvesting of the crystal is analogous to harvesting mako. Humans and their greed want to rip it from the Earth no matter the cost with devastating consequences. However, it is so useful and powerful that you understand why they're doing it. Certainly, it's the only thing that might help the Sioux stop the genocide of their people. Nevertheless, the book gives a kind of environmentally friendly aesop of, "Don't take the Lifestream crystals from the Earth."
I very much enjoyed the twists and turns throughout this book and strongly recommend it. No one's allegiances are entirely set and the only genuinely good person in the story is Mary Good Crow. Even she has a dark side that she struggles to keep suppressed as a matter of sheer survival. I think fans of the Weird West and urban fantasy both will enjoy this novel.