To listen offline, you'll need to do the following:
1. Add a title to your library.
2. Navigate to the library view.
3. Tap the 'download audio' icon.
4. The icon changes colour 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. Once it's successfully downloaded, the button icon changes from white to green.