To listen offline, you'll need to do the following:
1. Navigate to the library.
2. Tap the 'download audio' icon next the title you would like to download for offline listening.
3. The icon changes once successfully downloaded.
Please note: Downloading the audio titles is only available in the apps for mobile (smartphones and tablets) and not on the website. You can tell if an audio version is available by the little headphone icon on the book image.