Like a lot of other people, I enjoy the art books to animated films almost as much as I enjoy the films themselves.
That being said, some of the older Disney movies, like The Little Mermaid, were done before they put together the really juicy artbooks we see today, full of character and background designs, concept art, storyboard, etc.
So don't expect any of that from this. You'll get a cute little book mostly comprised of stills from the film. Definitely not worth the $50+ people are selling it for.
I would guess that if Disney does do a real art book for this film (and why wouldn't they?) they are going to wait until they release The Little Mermaid from the Disney Vault again for Blu-Ray or something, like they did for Beauty and the Beast and it's corresponding art book earlier this year.
Oh, and since there isn't an options for viewing inside this book here, you can see for yourself what it contains at this video: [...]