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.
Really enjoyed this book and it has enough twists and turns to keep me wanting to keep listening
listened to all three back to back, well written and read, not too cheesy, interesting concept of post apocalyptic world.