Nice little tie in to the rest of this series. Well worth listening to if you are listening to this series.
But be aware that the narrator is different from the main books. Nash sounds like an idiot stoner and it put me off a bit.
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.