Clay is an affable art-school graduate working as the night clerk at Mr. Penumbra's bookstore. He balances his curiosity about the unusual customers with his ability to apply modern technology to old problems, such as building a 3-D computer model of the shop. Rather than working alone, he gathers a diverse group of specialized friends to investigate the mysterious borrowing patterns he observes.

Mr. Penumbra is the enigmatic owner of the 24-Hour Bookstore and a long-time member of the Unbroken Spine fellowship. He occupies an intermediate position between tradition and modern progress, possessing both ancient encrypted texts and modern e-readers. He acts as a mentor figure who actively encourages curiosity in his clerks rather than demanding blind obedience.

Kat works in data visualization at Google and is fiercely ambitious, hoping to join the company's elite Product Management committee. She possesses an immense appetite for knowledge and is fascinated by the concept of the Singularity. She views the human brain as hardware awaiting a software upgrade, driving her interest in using corporate computing power to achieve life extension.

Corvina is the First Reader and head of the secret fellowship known as the Unbroken Spine, as well as the CEO of the Festina Lente Company. He is a strict traditionalist who fiercely opposes the use of modern technology to decode ancient texts. He wields significant power over his organization's members and demands complete adherence to centuries-old protocols.

Mat is Clay's roommate and a special effects artist working in the film industry. He prefers to build physical models with his hands rather than rely strictly on computer graphics, bringing an appreciation for manual craftsmanship to his art. He assists the investigation by fabricating physical replicas of important items.

Deckle is an attendant at the New York library who previously worked as a night clerk for Mr. Penumbra. He built the original database used to track the bookstore's inventory. Unlike many other members of the fellowship who dedicate their entire lives to secretive pursuits, Deckle maintains a family life outside the organization.

Oliver is a graduate student at Berkeley training to be a museum curator. He manages the daytime operations of the bookstore, handling the regular customers while Clay works the night shift. He uses his connections in the curation world to assist the investigation.

Neel is an initially unpopular high school student who grew up to become a wealthy and successful businessman. He shares Clay's deep love for fantasy novels and role-playing games. He eagerly agrees to act as the financial patron for Clay's investigations, treating the mystery like a real-world fantasy quest.

Lapin is a novice in the Unbroken Spine fellowship and a former computer programmer. She borrows heavily encrypted texts from the tall shelves at the back of the store, maintaining strict adherence to the organization's rules despite her technical background.

Tyndall is a novice in the Unbroken Spine fellowship who frequently visits the bookstore to borrow from the Waybacklist. He is deeply anxious about the strict rules of the organization and the severe punishments enforced by its leadership.

Ashley is one of Clay's roommates. She shares Mat's deep appreciation for physical details, which allows her to peacefully coexist with his expanding, scaled-down hyper-city project in their shared living room.

Grumble is a secretive, anonymous programmer who creates projects at the intersection of literature and code. He provides accessible digital tools to the public, including a portable cardboard book scanner and pirated typography fonts.

Eric is a stern associate of the Unbroken Spine who fiercely disapproves of the normal books at the front of the store. He acts as an angry messenger for the organization's leadership and reacts poorly to modern technological interference.

Moffat is a published author whose commercial fantasy novels contain deeply coded references to his own life. He is eventually revealed to have ties to the Unbroken Spine, using his fiction to document his experiences with the secretive fellowship.

Al-Asmari is Penumbra's former mentor who previously owned the 24-Hour Bookstore. He trained the current elders of the fellowship decades prior to the events of the story.

Gerritszoon is a historical fifteenth-century designer who invented a widely used typeface. He created original metal punches for printing presses and held crucial secrets passed down from the fellowship's founder.

Manutius is a fifteenth-century Venetian publisher and the founder of the Unbroken Spine. He printed Greek classics and authored an encrypted codex vitae that his followers have spent centuries attempting to decode.

Raj is a Product Management committee member at Google. He views physical books primarily as raw data that needs to be extracted and digitized for online access, demonstrating a highly utilitarian view of literature.

Finn is a Google employee who casually discusses the highly personalized corporate systems at their workplace, such as the company's automated food optimization processes.

Jad is the head of Google's book scanning project. He operates massive digitization machinery and assumes that physical bookstores are inevitably becoming obsolete as technology progresses.

Greg is a hardware engineer for Google who is simultaneously a novice in the Unbroken Spine fellowship, physically bridging the gap between the ancient book club and modern technology corporations.

Tabitha runs the California Museum of Knitting Arts and Embroidery Sciences. She provides Clay access to a massive museum database system known as the Accession Table to track down lost artifacts.

Cheryl is a friendly employee at the Consolidated Universal Long-Term Storage facility in Nevada. She helps guide Clay through the massive, automated storage warehouse to retrieve specific historical artifacts.

Trevor is a friend of Kat's who attends her social gatherings. His intimate conversation with Kat sparks jealousy in Clay while he attends the party remotely through a computer screen.

Imbert is a quiet, highly predictable customer at Mr. Penumbra's 24-Hour Bookstore who regularly borrows encrypted texts from the Waybacklist.

Federov is a solitary customer who participates in the mysterious borrowing patterns associated with the Unbroken Spine's collection at the 24-Hour Bookstore.