I could only say this, I'm addicted to this audio set. The consistency of the voices is amazingly acurate. I have to say she does a great job..
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've always found that the audio versions of the J.D. Robb books are more enjoyable to listen to over read due to Ericksen's narration.
I listened from start to finish !!!! Loved this book my new favorite until the next one comes out!