Have not read the printed version
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.
If you enjoy Garth Nix, you will love Jonathan Stroud. My two teenagers are delighted by these funny, creative, scary stories. I enjoy them too!