There are many reasons why a book might be unavailable.
Sometimes a book goes temporarily out of print - and sometimes no audio version has ever been recorded. Audible wants to give you the most complete selection we can and we'll keep adding series and filling in gaps as quickly as possible.
This is the second book in this series and I am hooked on them now. I know that they are kids books, but they are enjoyable and fun.
We have experienced only the audio edition. I think that it is more fun to experience that way, since there are so many different voices.