While most official tie-in books have been for younger readers, of late there's been more expansion into the young adult category, with novels delving into more complex and/or grown-up themes than the original films and television series did.
On this page I list the novels I'm aware of that touch the world of The Little Mermaid. There may well be others I've missed, so do let me know in the comments or by email!
The "Villains" books are a series by Serena Valentino that focus on one villain per book, presenting unique backstories and motivations for said villains.
Strictly speaking, these books aren't full-on retellings of the films from the villains' points of view. Instead, the series takes place in an entirely new universe where all these characters and all their stories coexist, with many of them encountering each other in crossover. (Not unlike the ABC TV series Once Upon a Time).
I haven't read the whole series, but from what I gather, none of the books are standalones, but form a continuous uber-story across the books. A great deal of this story is centered around original characters that connect the books together, with the stories of the individual villains happening on top of those "main" arcs.
Fairest of All (2009), The Beast Within (2014), Poor Unfortunate Soul (2016), Mistress of all Evil (2017), Mother Knows Best (2018)
The third book of the series is "Poor Unfortunate Soul", which depicts Ursula as Triton's long-lost sister who was raised as a human in Ipswich, by a human father who'd found her as a "shipwrecked" child. She only discovered her true heritage when the townspeople of Ipswich turned on her and her father, causing her father's death and forcing her to escape into the ocean with Triton, who'd finally come to claim her. But Triton ultimately rejected her for choosing an octopus form instead of a mermaid's tail, and ousted her from the palace. So Ursula's out for old-fashioned revenge, and enlists the help of the Odd Sisters, i.e. witches who are connected to the other books of the series.
It's an unusual backstory to say the least, and a lot of it is told in dialogue from Ursula to others, for the book opens with her already being an evil sea witch firmly on her path of vengeance with the Odd Sisters to help her. The events of The Little Mermaid take just about half the actual content with the book, with the rest of it being about the Odd Sisters and other new characters of the universe, so I was rather lost and missing context from the previous books.
There are some nice tidbits here and here: Ariel is portrayed sympathetically, Athena is mentioned as having been Ursula's friend and protector, the power of the ocean was split between the trident and the sea shell (just like in the stage musical), and there's a surprisingly deep cut canon shoutout of Harold the Merman. But ultimately the TLM characters aren't all that recognisable, and Triton in particular suffers the most. Which I guess tends to happen in villain-sympathetic stories... If the villain can't be all that bad, someone else has to be worse.
The "Twisted Tales" books are a series of "What-If" stories, four of them written by Liz Braswell and one by Elizabeth Lim. Each book posits that something in each film happened differently, and how that cascades into brand new problems that the characters have to face.
A book based on The Little Mermaid, "Part of Your World" came out in September 2018. Thanks to Colin for letting me know about it when it was announced!
In the "What-If" universe of "Part of Your World", Scuttle never discovered the connection between Vanessa and Ursula on the wedding barge, Ariel never got the kick to chase after Eric, and Ursula won the deal outright. From there Ursula's plan succeeded at every angle: she kept Ariel's voice, she got Triton to trade his life for Ariel's, and Eric stayed under her enchantment.
The book opens five years later, where Ursula has been living on land as Princess Vanessa, wife of Mad Prince Eric. Ariel, still mute, has become Queen of the Sea as penance for having caused King Triton's death, and although she still has the trident, every attempt to meet Eric and get revenge on Ursula has been blocked. What triggers the change in the new status quo is Scuttle, once again being a lynchpin: it is he who discovers that Eric still remembers Ariel in the recesses of his cloudy memory, and that King Triton is actually alive. So it's up Ariel to gather her wits and find a way to rescue not only Triton and Eric, but also Eric's entire kingdom that's on the verge of collapse thanks to Ursula's manipulations.
A Whole New World (2015), Once Upon a Dream (2016), As Old as Time (2016), Reflection (2018), Part of Your World (2018)
It's an exciting story that spins off from the original in unexpected ways. Some of the character choices may not be what I'm personally interested in, such as Ariel being forced by her sisters to be queen despite being the youngest (Attina, what are you doing!) and Ursula finding more joy as a land princess instead of lording over all the ocean, but author Liz Braswell put a great deal of thought into these characterization choices and shows her work in the text. She's drawn a direct, visible line from the film that we know to this slightly different world with an older Ariel and Eric, both of them traumatized and matured from their losses.
Like other spin-offs from the 1989 film, "Part of Your World" attempts to add layers to the canon by introducing new names, places, and lore in fleshing out the world. Ariel having the trident as Queen seems like a coup, until the book establishes that its powers are limited by the moon and access to the ocean. Ursula's powers are further explored, too, as is the greater human world around Eric's kingdom and his mysteriously unseen parents, and how exactly merfolk feel about humans and other animals (including seagulls like Scuttle!) eating fish.
“You've become a queen, a woman with a complicated personality. You have hidden depths and a wisdom and intelligence that all went unnoticed before by an idiot prince whose heart couldn't listen to anything his ears couldn't hear.” - Eric to Ariel
What about the characters themselves? It's a subjective thing, and the very concept of the book requires everyone to be older and wiser, so they can't sound the same as their original film versions. But I think Braswell does a really good job keeping the essence of the characters intact, eg. Ariel may be world-weary and far more knowledgeable than she used to be, but she's still kind, curious to explore and highly driven to do things her own way. As for the others, Eric is a big 'ol dork who gets some fantastic moments to shine; Ursula is wonderfully mad, bad and devious; Flounder, Sebastian and Scuttle are their loyal, adorable selves; and even Grimsby and Carlotta get a hand in the plot machinations. The new characters all feel like organic introductions, too.
I think what this book really does well is that it stays sympathetic to the high drama of the original. New media tends to criticise what came before it, but although the book acknowledges that Ariel made choices she shouldn't have under emotional duress, it never loses sight of her being Ursula's victim, nor does it belittle the passions she had as a young mermaid. Ariel and Eric's reuniting and rekindling their relationship under very different and much more honest circumstances is wonderful, especially for how it echoes instead of overriding the love story of the film.
There are other little moments throughout that are charming throwbacks to the animated canon, from how Ariel's sisters have always thought of her as "strange", Eric can be just as reckless as Ariel, Ursula still has a dangerous love of dramatics, and even a very clever shoutout to the mother Ariel was shown to be in The Little Mermaid II: Return to the Sea -- whereby this Ariel knows more about herself, and she won't make that mistake with her own child one day.