It didn't land in the Marketplace yesterday as announced, but Amazon's Audible audio book app for Windows Phone 7.5 is now available for download. This Metro-fied version of the app is free to install (including some short samples to get you started), and signing in with your Amazon account lets you purchase any books that the service has on offer. Media can be controlled using a swipe gesture interface, and there are even varying levels of badges to reward how much of a book-worm you become. That said,
we haven't been able to use the voice control functionally shown off by Microsoft at its Developers Summit is naturally a WP 8-only affair, so you won't find that feature here. If you're eager to get your literary listening on, you'll find a download and more info about the app at the source link below.
Update: As many of you have pointed out in the comments, the voice control aspect is specific to Windows Phone 8. We've updated our language in the post to reflect this.