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.
Its an intriguing story, and the narration is pretty good...
I like that it is getting more complex...and as always in a post apocalyptic world it is the other survivors you have to fear!
Always is, but I only listened!