I have 4 different libraries (one for each iPod), because we all have very different preferences. I forgot how I originally set them up, but to change libraries, you hold down the shift key when selecting iTunes. Then you can change libraries. To add a song from another library, you can add folder or add file to library (under the File option). I have two of the iPods set up to sync the entire library onto the iPod and the other two only syncs the checked songs. This set up keeps us all from scolling through songs we won't listen to (and keeps my kids away from my not kid appropriate running songs -- they have a good beat, but not appropriate lyrics).