Honestly don't have an order but if i did it would be pretty high up there
I especially loved the last two chapters as they were a continuation from where the first book ended.
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.