Sticking to Rex Stout's style would have made this better.
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.
The writing style is too different. The narrator is distracting. Fritz Brenner who speaks French is portrayed with a German accent - very annoying.