Android - How do you prevent Google Music from importing all .mp3 files on the device?

Well despite the comments about a .nomedia file not working in the newest version, on my fully updated Nexus 7 (as of April 2013) using a .nomedia file prevents the audiobook files from being scanned and listed in Google Music.

Some extremely simplistic testing seems to indicate that a single .nomedia file at the top of a directory tree is enough to prevent all sub-directories from being scanned. In the case I talked about in my question, a .nomedia file within the "audiobooks/" directory would prevent all books from being scanned, even if each book was a collection of .mp3 files within its own directory.

Adding .nomedia to the top level of a directory has worked for me, but also had to flip the source from "On device" to "All music". I am using Android 5.0.1 on an nVidia Shield Tablet.

I noticed that when I added .nomedia to my Podcasts folder (which I didn't want showing up in my Google "Play Music" app, it only disappeared once I changed the source from "On device" to "All music" and back again. Now it's working as described above. My Podcasts have been removed from my music player, but still show up in my Podcast player.