If you know you won't be able to stream from wifi and still want to keep up with your reading, downloading to your device is a great option. Downloading to your device is quick and easy. Go to your Library, select any title you've purchased and click the menu icon next to the title.
You will receive a menu from which you can choose "Download to Device". After selecting the option, you will see a progress bar indicating how much of the book has been downloaded. After the book has finished downloading, you will see an icon (currently a cloud) that indicates that the book is available on your device.