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 I could, I would of put emphasis on the opening crimes and put the Ballet behind the scenes.
In a word, boring. If you want an Alex Cooper novel, please choose one of the earlier ones.