Hi there and welcome
If you are just wanting the Kindle for Audio books and not for any services such as videos and games, then the Kindle DX iis the best unit for this based on price and functionality
It allows all Audio books to be played .
If you want additional functionality such as a colour screen and the ability to play Apps, videos, then I advise on the Kindle Fire, its more expensive though due to the additional functionality.
If you would like any additional information or assistance, please do not hesitate to let me know.
Else if you can take a second to rate my service by clicking one of the stars at the top of the screen then submit, that would be greatly appreciated.