How to Transfer an Audiobook to an iPhone
Audiobooks purchased from the iTunes store can easily be transferred to your iPod Touch and iPhone by syncing your device to your computer. Although iTunes has a tab for music, movies, podcasts and more in the iPhone window when your device is connected, it doesn't have a tab for audiobooks. Because of this you'll simply need to make sure your audiobook is checked for synchronization in your iTunes audiobook section located in the sidebar of the application.
Click "Start," "All Programs," "Apple" and "iTunes" to start iTunes in Windows. Click the "iTunes" icon on your dock to run the application on a Macintosh computer.
Plug the USB cable into the bottom of the iPhone and plug the other end into an available USB port on your computer.
Click the "Audiobooks" tab in the sidebar and be sure the box next to the audiobook is checked. Click the iPhone tab under the "Devices" section in the iTunes sidebar and scroll down to the bottom of the window in the "Summary" window.
Check the box next to "Sync only checked songs and video."
Click the "Sync" button and unplug the iPhone when iTunes displays the "OK to Disconnect" message.