A Letter to the Luminous Deep by Sylvie Cathrall

E. Is content with a solitary life in her extraordinary underwater home, until the discovery of a strange, beautiful creature outside her window, prompts her to begin a correspondence with a re-owned scholar, Henry Clel. The letters they share are filled with passion, at first for their mutual interest, and then, inevitably, for each other.

The Sunken Archive: Book one