Treehouse Books & Melanie Voland 
The Soul Cages: Traditional Mermaid Folk Stories Collection [EPUB ebook] 

A kind, shrewd fisherman, becomes friends with an Irish merman, called a merrow, and he visits his home under the ocean. But he soon discovers that the merrow is keeping something very strange, caged up there, and he feels that he must do something about it… A sweet, funny, sad, very mystical story, it contains subjects and themes that are not suitable for young children, but anyone else can read it. This is book 7 in our Traditional Mermaid Folk Stories Collection.