Best answer by Ryan E
If the downloaded audiobooks are free of DRM and in a supported audio format, then you should be able to play them using Sonos's Music Library function. However, if the audiobooks are only able to play through a proprietary app or are copy protected, then I'm afraid Sonos won't recognize the files and won't be able to play them.
If you have a Sonos player with Line-In, you can always wire your device (computer, phone, tablet) to the Line-In port and play the audio directly to Sonos that way. You can find some more information on Line-In here.
Let me know if you have any additional questions.