The Invisible Library book 6. Spoilers ahead for the earlier books.
As in, Irene and Kai are at a party of Lord Silver's being introduced as as the representatives for the treaty from the last book.
But then, when they return home, her parents have shown up. To show they are well, and to give her a message for a straightforward job from the Library. Irene must contact a Fae wheeler-and-dealer who has the only copy of a book they need to protect a world, and make a deal.
This involves a Caribbean lair, a government organization bent on controlling magical monsters, a museum that's closing, gambling with a dragon, sharks, a dragon who's been expelled from her family, someone stealing imperial regalia on a lark, a messenger traveling between worlds, and more.