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 love this series. Each book is funny and fresh and better than the previous novel. This is a most read series.
This is a terrific book in a terrific series. It is so funny and the plot keeps you guessing. I highly recommend it.