Fathomless by Jackson Pearce

Celia Reynolds is the youngest in a set of triplets and the one with the least valuable power. Anne can see the future, and Jane can see the present, but all Celia can see is the past. And the past seems so insignificant -- until Celia meets Lo.

Lo doesn't know who she is. Or who she was. Once a human, she is now almost entirely a creature of the sea -- a nymph, an ocean girl, a mermaid -- all terms too pretty for the soulless monster she knows she's becoming. Lo clings to shreds of her former self, fighting to remember her past, even as she's tempted to embrace her dark immortality.

When a handsome boy named Jude falls off a pier and into the ocean, Celia and Lo work together to rescue him from the waves. The two form a friendship, but soon they find themselves competing for Jude's affection. Lo wants more than that, though. According to the ocean girls, there's only one way for Lo to earn back her humanity. She must persuade a mortal to love her . . . and steal his soul.

I think Jackson Pearce has really found her talent.  Like her previous books this is a fairy tale retelling that kinda-sorta links in with the others but also works really well as a stand-alone.  It also really shines in comparison to the other two.  The characters in this are really crisp and have real depth.  I loved the dichotomy in Lo/Naida as the same person and also as two separate people.  Celia is also a great character and I like how she is similar to but different than her triplet sisters.  The plot is heavily character driven and really focuses on the emotional turmoil of Lo, Naida, Celia, and Jude which makes you feel their pain and gives the book a depth that absorbs the reader into the story.  If I had to find a fault I would say that the ending seemed a bit abrupt unless you had read Sweetly and Sisters Red and for its switch to action as a plot driver instead of characters, but Pearce does a good job of making things flow as smoothly as possible.  In all I thought it was a shining addition to Pearce's fairytale series and absolutely worth reading.