How would the player determine what is an audio book?
For example by directory. I store all my audio books in a single directory (and its sub-directories). The same organization I use for podcasts and language lessons.
Edit:
A while ago I also wrote a message to the mailing list: RE: NoDo or not?.
Yes, a great idea, but maybe not so easy to realize.
The simplest solution would be a hardcoded directory with some configurable properties.
Another solution could be a configuration file placed in the directory. Every time a file is played from a directory the software is looking for a configuration file and loads it if there's one. If there's no configuration file in a directory then load the "default" configuration.
Or implement it as a plugin, an audiobookplugin? Start the plugin and it shows the content of a directory, choose a file and it will play it with the given speed and maybe other audiobook specific settings. Quit the plugin and the normal settings are restored.