It's "The Little Mermaid, the Real Story". This book has two strengths: it is well-written and it is realistic. I know that "realistic" is an odd thing to say about a fantasy book, but I think it is appropriate.
The writing is handled professionally: smooth. unobtrusive, no typos or clumsy phrasing to interfere with the storytelling.
The author invented cultural norms and behaviors for the mer characters which feel authentic. In fact, the initial chapters are a bit like an anthropology text, providing the readers with an understanding of mer values and social structure. The rest of the book is a story of love, sort of, between two characters with different understandings of their relationship. The emotions and characterizations ring true. Again, the word "realistic" applies.