Thanks to the new integration with OverDrive's Libby app, you can listen to free audiobooks from your local library using Sonos speakers. When you download the Libby app, you will be prompted to register a valid library card. You can then connect the app to your Sonos system and use it to play available audiobooks. Progress is synchronized between devices, so if you're listening elsewhere, you can select where you left off.
Sonos' audiobooks aren't the only Libby (I'm watching audio), but unlike other services, they are completely free. The downside is that the choice of audiobooks is limited by what is available to the local library. Overdrive said Libby is working with 90% of North American public libraries.