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.
Strangely the historic aspect about Newark I didn't know about, some of his side descriptions sent me on a research marathon.
As always being a native of Newark the accuracy of the details of the sorroundings, and a strong story line.