“A letter to the luminous deep” is a mystery-romance with light academia tones. The peculiarity and absolute strong point of this novel is its epistolary narrative, which although at the beginning it is a little difficult to get along with many characters and unusual terms, the reading quickly becomes very fluent and interesting, while remaining too vague towards the underwater world where the protagonists live, whose nature and characteristics remain highly nebulous, and the whole story is extremely difficult to visualize in the mind, with the risk of losing the reader's attention and the grip on his imagination. This factor did not allow me to fully enjoy the book, also thanks to a mystery that was perhaps stretched out a little too much.