The seventh October Daye book. Spoilers ahead for the earlier ones.
Toby is searching the streets of San Fransisco, trying to establish what goblin fruit is doing. When they find a changeling who died of it -- it is safe for fey, instantly addictive and then rapidly lethal for changelings -- they stop for the night haunts to show up and claim her. This reveals that a dozen changelings have already died.
Despite her bad blood with the queen, she tries to appeal to her. It goes badly. The queen reveals she is the source and banishes Toby for being too much trouble. She has three days to leave the kingdom.
It's enough to send her to the Luidaeg for advice while still wearing the clothes that the queen had transformed on her back, only to discover that the Luidaeg is magically bound against telling her much. She can tell her some things, though, which starts her off.
Complications! Intrigue! Plot twists! (There was one plot twist I saw coming because things were entirely too easy, but only one -- and fairly early.) It involves fireflies, jam, armies from Undersea and Goldengreen and Shadowed Hills, discussions about the differences between treason, sedition, and insurrection, hobs cleaning up with enthusiasm, a room full of iron, and much more.