You can listen to audiobooks on the iPhone or iPod touch using the built-in audio/music player. You can purchase these books directly from the App Store or Audiobooks.com, and sync them to your device using iTunes. One cool trick is to adjust the playback speed faster or slower to suite your tastes.
To do this on the iPhone, go to the Home screen and tap on Settings >iPod >Audiobook Speed. On the iPod touch, go to Settings >Music >Audiobook Speed. Then select Slower, Faster, or Normal.