If online best to stream the audiobook. However, to listen offline and at a higher quality then download the audiobook. 

If you are on a limited data cell phone plan in would probably be better to use Wi-Fi. Please note that on some phones when you click the download link it might not seem as though anything is happening as the download is working in the background, so wait until the finished zipped file appears in the download folder.  

Instructions for opening the zip files on different phones, click on th applicable link: Samsung, Google, LG and other phones.
 Recommended:  android phone/tablet     mp3   256 kbit/s
  Slow internet, limited space: android phone/tablet   mp3 128 kbit/s