The fifth Invisible Library book. Spoilers ahead for the earlier ones.
After an initial adventure where Irene's sent to fetch a book, she sees Vale, whom Kai just happens to be visiting. That's where Bradament finds them with the news: incredible as it seems, the fae and the dragons are conducting peace talks in a Paris under the Library's watch -- and someone's been murdered. Vale's being hired, and Irene assigned as the Library observer.
Kai decides to just happen to visit his uncle, who's in charge of the dragon side.
It involves powerful fae, bound by their character type and with power to coerce others into playing roles in their stories; collecting witness statements; powerful dragons, whose moods cause the weather and earth to shift; another assassination mentioned earlier in the series; poisoned apples; knowing how to deploy a person as a character; anarchists; a book by Herodotus; and more.