Try deleting the problematic book (electing to remove original file if/when prompted) and then re-downloading the file from the iBook store.
You can re-download content purchased from the iBook store (availability varies depending on location) using the purchased option from the Quick Links section in the top right corner of the iTunes homepage in your iTunes application on your computer.
You can re-download content purchased from the iBook store (availability varies depending on location) using the purchased option at the bottom of the screen after you tap the store button in the top corner of the iBook app on your iOS device.
If the problem re-occurs, select the content which is causing a problem and use the 'Report a problem' button in Your Purchase History using your computer.