The following version of the book was used to create this study guide: Benedict, Marie; and Murray, Victoria Christopher. The Personal Librarian. Penguin Random House, New York, NY, 2021. Kindle AZW file.

Belle da Costa Greene is working in a library in Princeton in 1905 when she learns that John Pierpont Morgan, the famous financier, is looking for someone to take control of the vast collection of art and manuscripts he has collected. Belle returns to New York where her mother Genevieve is living in a small apartment with Belle's four siblings. Her older two sisters are working as teachers, and her brother Russell and sister Teddy are in school. The family is barely making ends meet, and Belle knows that landing this job could help the family's financial status. The night before the interview, Genevieve questions Belle about how she will act and what she will say. The preparation is necessary because Belle's family has a secret. They are light-skinned Black people living as White, and Genevieve often reminds Belle how important it is that she act in a way that never brings undue scrutiny.

Belle impresses Mr. Morgan, known as Pierpont, and he hires her for the job. She moves back to New York and begins to settle into the role. She quickly finds herself involved in the Morgan family's social circles as Pierpont has her attend parties and other functions. She begins to meet dealers and curators, often the only woman present for those conversations and auctions. She demonstrates that she is cunning, determined, and in control of millions of dollars as she chooses which manuscripts are most important to Pierpont's collection.

There are a few negative points in her life, including that she knows she can never marry and have children for fear that a dark-skinned child will give credence to the rumors that sometimes circulate about her. Genevieve created a Portuguese grandmother to explain the swarthy complexion, but Belle knows she has to be careful with her secret – especially around Pierpont and his peers. Being a woman in the art community is already difficult, but a Black woman would be immediately and completely ostracized.

Belle has a chance encounter with Bernard and Mary Berenson. Bernard is an expert and author in the art world and Belle finds herself pulled toward him. Bernard and Mary are married in name only, and Mary encourages their affair. Belle is torn when she discovers she is pregnant, wondering if she might have this child with Bernard. He puts an end to that fantasy, demanding an abortion and then leaving her alone when she is hospitalized with an infection. Though they reconnect briefly a few years later, Belle ends their relationship when Bernard reveals details about the Pierpont collection to an art dealer.

Belle and Pierpont continue to work closely together for years, and they carefully avoid the romantic pull each feels. After a particularly important purchase, Pierpont kisses Belle. Though they take it no farther, Belle feels their relationship is changed from that time. Pierpont leaves the country for a trip to Europe, and Belle is devastated when she receives a telegram saying Pierpont died. She grieves over him but also worries about her future and the future of his beloved library.

Jack takes over and soon reveals that he plans to sell off a few pieces of the collection, but he keeps the bulk of it intact. They work together, as Belle and Pierpont had but without the sexual attraction. Eventually, they open the library to the public. Belle continues to oversee the program. In 1948, she looks back on her life as she burns papers and letters that would provide her true identity.