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.
Yes, the reader narrator is so good, easy to listen to and creates a mental picture of story and each character.
Narration is always great in this series. It's like you are watching the show. you can picture the entire set.