TOP 50 (I HAVE 512)
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 only keep a small amount of books on my phone bc it has only so much memory. This is one of the books I keep.