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.
This is a dramatization of the story. I have neither read nor listened to the original story.
Always a good combination: Le Carre and a BBC dramatisation.
Worth a listen. A lot going on, though not the most cohesive Le Carre plot.