Stranded in a mysterious underwater city, E. and Henerey navigate a society obsessed with knowledge while deepening their tender romance and confronting their own mental health struggles, as their siblings Sophy and Vyerin search for a way to reunite with them. Told through letters and records, this epistolary fantasy explores themes of found family, the complexities of love, and the tension between academic curiosity and cosmic danger.

Reader Review Summary

Set in a magical underwater world, this novel concludes the Sunken Archive duology and is told entirely through an epistolary format, using letters, journal entries, notes, and records to convey the story. The narrative follows E. and Henerey, former correspondents who find themselves stranded in an enigmatic underwater city after passing through a mysterious Structure in E.'s garden. As they adapt to a society obsessed with knowledge and face the possibility of never returning home, their siblings Sophy and Vyerin search for elusive Entries to rescue them, all while an ancient cosmic threat looms. The book explores themes such as mental health, the complexities of love and found family, the dark side of academia, and the tension between perception and reality. The world-building includes underwater cities, domed libraries, and a society of scholars with unique customs, such as arriving hours early for appointments and suppressing emotional expression. The story is multi-POV, with new characters like the Thirtieth Second Scholar and Lady L. introduced, and the plot unfolds across different timelines and perspectives.

Readers appreciated the gentle, compassionate tone and the focus on character relationships, particularly the romance between E. and Henerey, which is depicted as slow-burn, sweet, and supportive, with both characters managing mental health challenges together. The representation of mental illness and LGBTQIA+ identities, including asexuality, was frequently highlighted as realistic and respectful. Many found the prose beautiful and the epistolary structure engaging, enjoying the process of piecing together the narrative from various documents. The underwater setting, scholarly society, and whimsical world-building were praised for their detail and atmosphere. Some readers also noted the inclusion of a helpful recap at the beginning, the development of E.'s character, and the addition of new, amusing characters like the Thirtieth Second Scholar. The found family dynamics, especially between Sophy and Vyerin, and the exploration of grief and love in different forms were also cited as strengths.

Common criticisms centered on the pacing and structure, with many readers finding the book slow, repetitive, or lacking in momentum. Several noted that the plot took a long time to develop, with significant revelations and action occurring only in the final chapters, making the earlier sections feel like a 'slog' or 'chore.' The epistolary format, while appreciated by some, was seen as contributing to a lack of distinct character voices, with letters often sounding similar regardless of the writer. Some readers felt that the narrative was too focused on day-to-day interactions and expressions of affection, at the expense of plot progression or character development beyond what was established in the first book. The resolution of the central mystery and the cosmic threat was described as abrupt, underwhelming, or lacking in emotional impact, and some wished for more integration of historical documents throughout the story rather than in a single section. A few readers struggled with the formal, academic tone, which created distance from the characters, and some did not finish the book due to lack of engagement.

The book is not a standalone and requires familiarity with the first volume, A Letter to the Luminous Deep, for full understanding. The narrative is multi-POV and alternates between timelines, with some readers finding the structure challenging to follow. The story includes both romantic and platonic relationships, with a focus on found family and the process of self-discovery. The society depicted in the underwater city is characterized by strict rules, emotional restraint, and a preoccupation with knowledge, which influences the interactions and conflicts among characters. The duology as a whole is noted for its commitment to the epistolary format, and while the conclusion provides closure for the main characters, some readers expressed a desire for an epilogue or further exploration of the world and its history. The book was published by Orbit Books and is marketed toward adult readers interested in fantasy, romance, and LGBT themes, with an average reader rating of 3.96 out of 5 based on 344 ratings and 159 full reviews.