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.
After Glow was like part 2 of After Dark. It finished up some story lines and started some new ones. I love the Harmony series.
Jayne Castle, sure. Rosenblat, nope.