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.
Have not read the print version.
Wait until it's on sale. Love the series so I got it on sale. The story isn't one of the best but I like to get everything in a series.