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.
I really enjoy Imogen Church's narration. Listened over 2 days whilst doing a very boring job. Helped pass the time admirably.
Really enjoy this author's books. Very easy to listen to, great suspense and a sense of familiarity with characters.