@pluralistic re: "Not surprisingly, a lot of people assume that if an Audible search for an author or book comes up empty, that means there is no audiobook available. They don't think of searching for the book on Google Books, or Libro.fm, or Downpour."
Is it not possible to write an app that searches these alternative sites, provides hooks into each for purchasing, and downloads the books, all in a seamless, unified interface? Something like a federated marketplace app?