Return to the world of the Invisible Library for Irene's most perilous mission yet . . .
Librarian Spy Irene is heading into danger. Not for the first time, but could this be her last? She's tasked with a terrifyingly dangerous solo mission to eliminate an old enemy, which must be kept secret at all costs. But even more troubling news emerges. Multiple worlds are disappearing - and the Library may have something to do with it.
Determined to uncover the truth behind the vanished worlds, Irene and her friends must descend into the unplumbed depths of the Library. And what they find will change everything they know. This may be Irene's most dangerous assignment of her hazardous career.