My favorite compiler of all things fairy tale related, Heidi Anne Heiner, has a mermaid/water sprite book coming soon!
her blog! You can also check out the table of contents for this 828 page book! Also be sure to pick it up as soon as its out, if the response is good enough she's got enough info to write a second volume!!!
You should also pick up her other books! There's a story in her Sleeping Beauties book called Richilda (it's a Snow White tale) that knocked my socks off!