Audible allows you to listen to audio books on your mobile devices. It is another company owned by Amazon (a business I really, really like for many reasons) so purchasing audiobooks is really simple. You can either opt for subscription plan or purchase them as one offs.
Thankfully Audible has finally come to Windows Phone 7 and can be downloaded for free:
I have for a long while now used audio to listen to podcasts, books, etc while on the go. It allows to to get through so much more stuff. Once you start you’ll never stop, trust me. Sign up for Audible today and expand your mind.