Apple slipped a nice bonus into their iTunes 7 upgrade: The abiliity to easily manage multiple music libraries.
Switching libraries is simple; hold down the option key on your Mac (or shift key in Windows) when you launch iTunes. You’ll be presented with an option to create a new library or choose the library you want to load instead.
This is a wonderful feature if you want to seperate a collection of audio books or kids music from your main iTunes library.
Note: In my own case, Sandra Boyton’s Penguin Lament made the jump from the kids library into the main iTunes library.