When this book was written, in August 2018, Android Auto was not open to all developers. The Android Auto application was only available in the marketplace to tier-one approved vendors, such as WhatsApp, Spotify, Waze, Pondora, and ABC News. You can easily check out some of the available Android Auto apps by going to Android Auto | Apps for Android Auto.
The following image shows the Android Auto applications:
Google has not yet to announce when the Android Auto development will be open to all developers so that third-party applications can be created and available for all Android Auto users. But it is important to be on the breeding edge and at least develop Android application in development mode and learn to interface with Android Auto now, so that when the Android Auto marketplace is open, you will be way ahead of your competition...