The media playback situation on Android Auto is not great, but it's a little better today now that Audiobooks.com works in the car. This is somehow the first audio book source with proper support for Google's car platform. Hey Audible, maybe you want to get on that too? Thanks.
The app has the same basic UI in the car as other Android Auto media apps. There are playback controls, cover art, and a nav drawer on the left. The nav drawer in this app offers access to your downloaded books, featured samples, and free books. The only seek options are jump forward/back 30 seconds, though.
Audiobooks.com has over 60,000 titles in its library, some of which are free. If you sign up for a subscription ($14.95), you get one paid book per month and can purchase more at any time. There's a free trial with one included premium title as well.