Laura is a dedicated wife and mother living in a seven-room apartment hidden inside the New York Public Library in 1913. Craving intellectual stimulation beyond her domestic duties, she enrolls in the Columbia University School of Journalism. Her reporting assignments lead her away from typical high-society stories and into the gritty, politically active world of Greenwich Village, awakening a desire for personal and societal change.

Sadie is a bookish, introverted librarian who works with the New York Public Library's rare Berg Collection in 1993. Recently divorced, she prefers the predictability of historical artifacts over human interaction. When she is suddenly promoted to temporary curator amidst a series of high-profile book thefts, she must protect her career while hiding the fact that her grandparents were suspects in similar thefts eighty years prior.

Jack is the superintendent of the New York Public Library in 1913. He manages the sprawling facility while attempting to write a novel in his spare time. Although he initially supports his wife's educational pursuits, he maintains traditional views on gender roles and expects Laura to prioritize their children and home over her journalism career.

Amelia is a physician managing a city-funded program that provides pediatric care to impoverished mothers. She is a former Vassar classmate of Laura's who embraces a bohemian lifestyle in Greenwich Village. Unapologetic and authoritative, she introduces Laura to the Heterodoxy Club, fundamentally altering Laura's perspective on women's rights.

Nick is a security consultant hired by the library board to investigate a sudden wave of manuscript thefts in 1993. He collaborates closely with Sadie, treating her as a valuable asset rather than just an employee. Beyond his professional exterior, he enjoys salsa dancing and poetry, slowly drawing Sadie out of her shell.

Harry is the young son of Jack and Laura Lyons. Sensitive and struggling to read, he feels neglected as his parents become increasingly absorbed in their respective writing ambitions. Left to his own devices, he begins skipping school and roaming the city streets, seeking attention and guidance elsewhere.

Robin is the babysitter for Sadie's niece, Valentina. She spends time in the family's home, quietly observing their dynamics and bonding with Sadie over the shared difficulty of managing aging parents. She grew up in Massachusetts with a difficult childhood and finds solace in literature.

Pearl is the daughter of Jack and Laura Lyons who grew up in the New York Public Library apartment. By 1993, she is elderly and declining from dementia. She holds tightly to fragmented memories of her childhood, dropping cryptic hints about stolen books that frustrate and intrigue her daughter Sadie.

Dr. Hooper is the director of the New York Public Library in 1993. Tasked with managing the institution's public image and finances, he is highly anxious about the upcoming fundraising campaign. The string of book thefts forces him to make difficult personnel decisions to protect the library.

Claude is a staff member working in the Berg Collection in 1993. He possesses longer tenure than Sadie but is passed over for the temporary curator position. This professional slight is compounded by the fact that Sadie recently rejected his romantic advances, creating a strained working environment.

Lonnie is Sadie's brother and a stabilizing force in her life. He shares his home with his wife, daughter, and their ailing mother, Pearl. He provides practical advice to Sadie and serves as a sounding board as she uncovers clues about the family's library connection.

Valentina is Lonnie and LuAnn's young daughter. She is highly observant of the adults in her home, especially her grandmother Pearl. She listens carefully to Pearl's fragmented memories and often relays important details to Sadie.

LuAnn is Lonnie's wife. She manages their busy household while dealing with the challenges of her mother-in-law's dementia. She is observant of Sadie's emotional state, gently suggesting that Sadie uses historical research to avoid processing personal grief.

Dr. Wakeman is Laura's professor and thesis advisor at the Columbia University School of Journalism. He initially praises her investigative skills but maintains strict, discriminatory standards regarding what topics female students are allowed to cover.

Dr. Anderson is Jack's superior at the New York Public Library. He recognizes Laura's potential and actively supports her educational ambitions by writing a strong recommendation and securing a scholarship for her first semester.

Edwin Gaillard is an investigator assigned to look into the 1913/1914 rare book thefts at the library. He questions the library staff regarding their finances and access to the locked cages.

Red Paddy is the leader of a group of older boys who roam the city streets in 1913. He exhibits antisocial and disrespectful behavior, alarming Laura when she learns her son has been spending time with him.