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 title or image. The first time you select a title you will be asked if you wish to download or stream the audiobook.
If you do not wish do download it immediately, you can always choose to do so later by click the audiobook menu icon from the library or now playing page.
You will receive a menu from which you can choose "Download to Device". After selecting the option, you will see a percentage indicating how much of the book has been downloaded. After the book has finished downloading, the cloud icon will disappear which means the book is now stored locally on your device.