If you want to transfer music on two or more computers(libraries), I have found a trick/solution!
1. simply connect your iphone to any other comuter and open itunes
2. open explorer or finder and find the music u want to copy onto iphone/idevice
3. select the music u want then press ctrl-c
4. go back to itunes and select your idevice
5. click on music( in your idevice)
6. press ctrl-v
vloila it starts copying them over.