I've recently accepted a summer reading/listening challenge to discuss C.S. Lewis' book Mere Christianity. I've only started to listen to the audiobook, which suits my busy summer much more than reading at this time.
I found you can download a FREE mp3 copy of Mere Christianity here. You should be able to right click & save. If that fails go to main page which is here. Save to your iPod or CD & listen, pause, rewind anywhere/any time.
You can buy a cheap paperback on Amazon.
C.S. Lewis, a former Atheist, does a brilliant job of explaining in clear & concise analogies the world, and specifically the Christian faith.