Alexandria “Alix” Watson has learned one lesson from her barren childhood in the foster-care system: books will never let you down, unlike people. 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 and bookmarking their way from volume to volume, 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 their enemy draws closer. But who does their enemy really wish to destroy—the Library, the Librarian, or Alix herself?