In all the world, there was only one place humans could get wishes: Brimstone's shop. And there was only one currency he accepted. It wasn't gold, or riddles, or kindness, or any other fairy-tale nonsense, and no, it wasn't souls, either. It was weirder than any of that. It was teeth.[loc. 460]
Karou is a seventeen-year-old art student in Prague, smarting over the treachery of her ex-boyfriend Kaz and enjoying city life in the company of her friends. She is also errand-girl to Brimstone, a kindly monster who collects teeth -- or, rather, has Karou collect them for him -- and creates wishes out of them. Wishes aren't exactly magic, but they bestow powers: the more powerful the wish, the greater the chance that it might go awry. Brimstone, meanwhile, won't tell Karou anything about her origins, or about his own purpose: but he does give her a new language, wish-granted, every birthday.
Karou has more or less resigned herself to happy ignorance when she encounters Akiva, a beautiful and dangerous young man who has been sent to destroy Brimstone's workshop.
As Akiva and Karou get to know one another, they both learn of the ancient war between the seraphim and the chimaera -- and of their own roles in that war. For Karou's name means 'hope' in the chimaera language: and Akiva's hands bear the tally-marks of all the chimaera he has slain.
There are rather too many explanations and infodumps in this novel, the first of a trilogy: but that is a reflection of the complexity of the world-building and the characters' backstories. Karou's adoptive family of 'devils' -- and the questions she's never thought to ask about them -- contrast sharply with the beautiful, terrible seraphs and the centuries-old war that consumes them all.
The rules of magic in this universe are harsh: power comes from pain, and it need not be one's own pain. (The most powerful of the wishes that are crafted in Brimstone's workshop are those which are paid for with one's own teeth, self-extracted.) And when one individual can gain from another's pain, the result is slavery.
I'm looking forward to reading the other two volumes in this trilogy, though I am faintly disquieted by the relationship between Akiva and Karou: perhaps when Karou is more fully herself, and has assimilated her own past, things will feel more balanced.