You can download the Audiobook Mp3s and play them on your iOS mobile device but you will need to first download the files to a computer, upload them to iTunes and then sync them to your device.
Please see these articles for more detailed steps:
http://ipod.about.com/od/addinganddeletingsongs/ss/adding_mp3s.htm
https://support.apple.com/kb/PH20507?locale=en_US