Does anyone know where to start, or even if you can write add-ons for managing extras like audio books or your own collection management routines / visualizations?
The Zune software hasn't a plugin interface. Though there are some programs that read the registry data set by the Zune software and parse it. One example is the Zune - Now Playing. the address is http://znp.codeplex.com/. Hope it helps you in some way.