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.
So far the series is very good in all aspects. Writing. Reading. Are very good.
Very enjoyable story. Great narration. Very little to dislike. Couldn't stop listening and can't wait to listen to the next one.