Sorry, but I don't actually know if this is an app problem, an android problem or a Graphene OS problem. I've tried a lot of different audiobook players, and the one I really like is Voice Audiobook Player. https://play.google.com/store/apps/details?id=de.ph1b.audiobook
It lists Android Auto as one of its features, but it doesn't show up at all in the Auto app list. How can I go about figuring this out?
Voice Audiobook Player - Android Auto
Bronzing7407 did you download it from the play store directly? If not you have to enable unknown sources in Android Auto.