Undine

Undine is a fantasy by Friedrich de la Motte Fouqué written in 1811. Wikipedia notes the book

is descended from Melusine, the French folk-tale of a water-sprite who marries a knight on condition that he shall never see her on Saturdays, when she resumes her mermaid shape

In this version of the story, the water creature is sent to a human family as a child so she can grow up and marry a human and thus gain a soul.

Wikipedia names many adaptations of this book to dance, music, art and in other works of literature. Of particular interest to me are the 2 film adaptations:

1. Andy Warhol very loosely adapted it for film in 1968 and re-made Ondine into a gay man; and 
2. Ondine (2009), with Colin Farrell, which looks to be a straight drama with no fantasy elements at all.

The book is available online. It can be read online here with the Arthur Rackham illustrations, one of which is at the top of this post. You can listen to the story at LibriVox.

This makes an interesting read. Absorbing, as most fairy tales are to me, and I didn't find the slightly archaic language at all distracting. It begins:

Now it may be hundreds of years agone that there lived a worthy old fisherman, and he was seated on a fine evening before his door, mending his nets. The part of the country where he lived was right pleasant to behold. The grassy space on which his cottage stood ran far into the lake, and perchance one might well conceive that it was through love of the clear blue waters that the tongue of land had stretched itself among them; while with embrace as close and as loving the lake sent its arms round the pleasance where the flowers bloomed and the trees yielded their grateful shade. It was as though water welcomed land and land welcomed water, and it was this made both so lovely. But on this happy sward the fisherman and his household dwelt alone. Few human beings, or rather none at all, even cared to visit it. For you must know that at the back of this little tongue of land there lay a fearsome forest right perilous to traverse. It was dark and solitary and pathless, and many a marvelous strange creature and many a wraith and spectral illusion haunted its glades, so that none might dare adventure unless a sheer necessity drave them.

Nathless, the worthy fisherman might pass unharmed, whensoever he was carrying some choice fish caught in his beautiful home to a large town bordering the confines of the forest. He was a man full of holy thoughts, and as he took his way through the gloomy shades peopled with forms of dread, he was wont to sing a pious chaunt with a clear voice, and an honest heart, and a conscience void of guile.

Sometimes when we see or read things around here, we say that Klingons would like them since, "Lots of people die, and nobody makes a profit." This book qualifies.

I read this as part of the Once Upon a Time Challenge.