This is one of those books I actually had to think long and hard about after finishing it because I was so conflicted over how to rate it, since there quite a few things I enjoyed but also at least as many I was frustrated with.

Starting with the positives, I was immediately hooked on the premise for this book: a light academia epistolary underwater fantasy mystery romance sounds like a mouthful, but also exactly like the type of book I would enjoy. Told entirely through letters, the book revolves around the disappearance of reclusive E. and scholar Henerey Clel following a mysterious discovery they made, and their siblings' attempts at making sense of this.

The relationships that develop between the different characters, which readers can witness slowly unfurling through their correspondence, were by far my favourite part of the book. Interactions between the different characters are realistic and rich, whether that's the sisterly relationship between E. and her Sophy, the budding romance between Henerey and E., or the friendship between Sophy and Henerey's brother Vyerin, bound together by grief and the need for closure. It is hard to convey such deep levels of human connections only through letters, but the author did a wonderful job of it.

The world is also intriguing and lavishly built, with many natural wonders and sea creatures being thoroughly presented throughout the book as the different characters engage in their research, and I was keen to learn as much as possible about them. What comes across as less clear in the world building is the way society is structured and functions, despite references to academia and its structure throughout. Now, it is entirely possible that this will become clearer in the next book (or books?) in the series, since this is only the first one - something which I realised very late on, as for some reason I was convinced I was reading a standalone.

What I struggled most with in reading this book were the writing style and the pacing. As the characters are all scholars or researchers, their use of language reflects this, with a prose reminiscent of Victorian English laced with academic jargon. This isn't typically an issue for me, since I'm used to reading academic papers for work, and I enjoyed other books written in a similar style: for instance, Emily Wilde's Encyclopaedia of Faeries is narrated through research journal entries and I had a great time with it. With A Letter to the Luminous Deep though, the sparks just didn't fly, and I ended up finding the style verbose and overly weighed down by unnecessary formalities, even in letters between siblings.

I was also completely unable to distinguish between the different voices, as all the characters sounded exactly the same in their writings. At one point, I had to switch to the audiobook version (which is excellent, by the way), as the production featuring multiple narrators finally allowed me to understand whose letter I was reading without constantly having to go back to check. This made me feel as though the book was just dragging along, something which wasn't helped by the extremely slow pace. Now, I generally prefer character development to plot so maybe this was a case of me reading this book at the wrong time, but I really struggled to get through it for large sections in which it felt as though nothing at all was happening. And then, everything happened all at once, ending on a pretty huge cliffhanger, which I found pretty frustrating.

I think I'll probably pick up the next book in the series since I am somewhat curious as to where the story might go next. This is also a debut and I see some great potential for the author in the next books, especially given how the relationships in this book were some of the best written I have come across recently. I also liked the queer normative society and the mental health rep (with depictions of OCD and anxiety), so there's definitely a lot to save here!

I received an advanced review copy of this book for free from the publisher via NetGalley. This did not affect my opinion of the book in any way.