Alexandria “Alix” Watson has learned one lesson from her barren childhood in the foster-care system: unlike people, books will never let you down. Working three dead-end jobs to make ends meet and knowing college is a pipe dream, Alix takes nightly refuge in the high-vaulted reading room at the Boston Public Library, escaping into her favorite fantasy novels and dreaming of far-off lands. Until the day she stumbles through a hidden door and meets the Librarian: the ageless, acerbic guardian of a hidden library where the desperate and the lost escape to new lives…inside their favorite books.

The Librarian takes a dazzled Alix under her wing, but before she can escape into the pages of her new life, a shadowy enemy emerges to threaten everyone the Astral Library has ever helped protect. Aided by a dashing costume-shop owner, Alix and the Librarian flee through the Regency drawing rooms of Jane Austen to the back alleys of Sherlock Holmes and the champagne-soaked parties of The Great Gatsby as danger draws inexorably closer. But who does their enemy really wish to destroy—Alix, the Librarian, or the Library itself?