Undine

It is a romance book. Undine is a fairy-tale novella (Erzählung) by Friedrich de la Motte Fouqué in which Undine, a water spirit, marries a knight named Huldebrand in order to gain a soul. It is an early German romance, which has been translated into English and other languages. During the nineteenth century the book was very popular and was, according to The Times in 1843, 'a book which, of all others, if you ask for it at a foreign library, you are sure to find engaged'. The story 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. It was also inspired by works by the occultist Paracelsus.