Plot
After being orphaned by a sabber attack on her tiny village in the Dust, Livira is taken to Crath City by Malar, a swordsman who was among the men who rescued her. Though she faces discrimination from the city residents, she is taken under the wing of Davris Yute, an important librarian who ensures she is educated by the library staff. The library is composed of many enormous chambers, many of which are locked or accessible only with the help of automaton-like Assistants left over from the time the library was created.
Evar, meanwhile, lives in an isolated chamber of the library, unable to leave or interact with anyone other than his adopted siblings, the automaton-like Assistant who has raised them, and the implacable Soldier who protects them. Evar and his siblings were each taken from their own time while using the Mechanism, a device inside the library that lets people enter the world of whatever book they carry. Though the siblings did not age during their decades trapped in the Mechanism, each became an expert on the subject of the book they were trapped with – Kerrol in psychology, Starval in assassination, Clovis in warfare, and Mayland in history, though he has been missing and presumed dead for more than a year. Unlike his siblings, Evar does not remember what book he took into the Mechanism or how he spent his time there. Periodically, the Mechanism sets loose Escapes, beings of dark energy that form as a byproduct the machine. While hunting an especially dangerous Escape, Evar finds a book with a handwritten note addressed to him alongside instructions to "find me at the bottom."
Evar realizes the note must be referring to the well that is the source of the library's water. He dives into the water and is taken to the Exchange, an in-between space that allows travelers to move between libraries across worlds and times. While in the Exchange, he finds a young Livira, who stumbled across another portal in her own library while exploring. They are quickly separated, and though Evar attempts to reach her again, her portal is closed. When he experiments with another portal, he is transported to the past at the moment when sabbers slaughtered Clovis's family. When he returns to the Exchange, he meets a teenage Livira. Though little time has passed for him, she has been studying in the library for years, seeking a way back to the Exchange. Though they share a longer conversation, Escapes begin attacking the Exchange, and Livira flees back to her own library while Evar jumps into a new one. This time, he watches Crath City be overrun by sabbers after a long siege. Sick of the violence, he returns to the Exchange, again finding Livira, who this time is his own age.
In her time, Livira has continued to rise through the ranks of the library. Outside of Crath City, the sabbers have become an even greater threat, encroaching on human land as they flee from an unknown enemy. Crath City's king and political leadership call on the library to provide texts that dehumanize the sabbers and equate them to animals. Yute, her mentor, urges her to consider that the library brings both blessings and curses. Malar, now in the employ of Yute, saves her from several assassination attempts motivated by Livira's heritage in the Dust. She begins to write her own book, though such creations are forbidden by the library staff. During her quest to understand the library's history and return to Evar, she also encounters the head librarian, a mysterious figure with some unknown connection to Yute.
After years, Livira discovers a way to create a portal to the Exchange, where she meets Evar for a third time. Evar and Livira use a new portal together, seeking a time without bloodshed. They find an empty version of Crath City from years before. While exploring, their connection deepens and they share a kiss for the first time. When they enter a building, however, they see the corpses of both humans and sabbers, apparently killed in a massive chemical attack.
Both horrified, they exit into the Exchange again, where they are immediately attacked by Clovis, who sees Livira as a sabber. Evar and Livira escape to their respective times, realizing that the Exchange created an illusion that hid the truth of their identities. Livira is a human, like the ones who slaughtered Clovis's family, while Evar is a canith, like the ones laying siege to Crath City, which in fact has passed back and forth between the two species for generations.
At Livira's return to her own library, the canith invasion begins, and she flees into the library with her friends and the head librarian, who is revealed to be Yute's wife and an Assistant who, like Yute, gave up her immortality in order to impact the world. In the fighting, both sides use weapons developed through library research, causing a massive firestorm. Evar, returned to his own time, struggles to explain what has happened. The siblings are reproached by the Assistant, who forbids them from using the Exchange. Nevertheless, Evar and his siblings create a distraction and return to the Exchange, where they encounter the humans that Livira has led there while escaping from the fire. The two groups almost begin the bloodshed again, but Livira and Evar reconcile and convince their groups not to attack.
During the standoff, Yute explains that they are all part of a larger conflict taking place between forces that support and oppose the library. At a critical moment, Mayland, the missing sibling, emerges from one of the portals and accuses the head librarian of taking the wrong side, executing her. Simultaneously, Evar and Livira discover that the book Evar brought with him into the Mechanism is the story that Livira has been writing. Livira, Evar, and Malar then use the Exchange to return to the burning library, where Livira and Malar inhabit the bodies of Assistants in order to save their friends. Evar realizes that the Soldier and the Assistant who raised him are in fact his two friends, though their minds and actions have been restricted by the rules of the library.
Evar returns to his library, where the Assistant and the Soldier have been nearly destroyed by a new species, called the skeer, who have entered the library. Though he defeats the remaining skeer with the help of Clovis and his other siblings, their two guardians appear to be destroyed. Unbeknownst to Evar, however, the physical destruction of the Assistants has freed the spirits of Livira and Malar, although they have no form. The siblings pass through the door used by the skeer, with Kerrol promising Evar that he knows a way to find Livira.
