Irene, agent of the Invisible Library, is now well established in an alternate London as the Resident Librarian. Together with her apprentice, Kai, she attends a dubiously legal auction to acquire a copy of a not actually illegal book for the Library, annoying some people in the process.
When they are waylaid on the way home, they're puzzled by several aspects of this attempt to, apparently, get the book--especially by the fact that, when overcome and questioned, the thief admits to being sent by a member of the Fae, associated with the Lichtenstein embassy. It gets stranger when Lord Silver, an old adversary among the Fae, the next morning gives them an oblique and confusing warning about possible danger.
They take their limited information and set out to investigate, working separately to cover the most ground. But they've made a mistake; they've misunderstood who the threat is to. It's not Irene, either personally or as a representative of the Library, who's at risk.
It's Kai, son of a dragon king, And when Kai is lured into a trap and carried off deep into Chaos, Irene has to find and rescue him--and that means working with Lord Silver, who is no friend, but who may be, for certain limited purposes, a temporary ally.
This is a really fascinating fictional universe. The Fae represent Chaos; the dragons represent Order. Too much of either isn't good for ordinary human beings--and the Library works to maintain a stable balance between the two, in part by collecting in the Library important books from all alternate realities. The Language of the Library is the language of truth, and with it Irene can command anything non-living--with appropriate focus, and knowledge of their proper names. To some extent, she can also influence people, to a limited degree, for short periods of time, with the expenditure of a lot of energy.
None of this really prepares Irene for the alternate Venice where Kai is imprisoned, nor does it help her in dealing with Kai's rather angry uncle--another dragon king.
I bought this book.