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.
Once again Harlan Coben nails it. I can't wait to start the next book in the continued story.
As always a fantastic book by Harlan. I hope he writes more for these characters soon!