Dan Sanderson provides a clear explanation, including pretty good photos, of one of the user interface “quirks” that can get you when using the iPhone for audiobooks, namely, the iPhone’s indicator and toggle buttons for Repeat and Shuffle modes are somewhat hidden, not 100% clear, and can be toggled accidentally if you’re not careful.
When listening to audiobooks, you of course want both Repeat and Shuffle turned off. On standard iPods, you do this in the main settings menu, and it’s effective for all tracks. But on the iPhone, as Dan explains and illustrates, the setting is harder to find, until you know where it is, and can be turned on and off while fiddling with your place in a track. See Dan’s full post for the details.