One digital life has a straightforward how-to on using iTunes to backup your music library, which can span across multiple discs. For immediate level of iTunes user, he has a way of backup any new musics as well without spending time to manually track them:
Now, you’re going to be adding more music to your library at some point, right? You don’t want to have to backup everything again do you? Tracking new additions to your library is a snap. Simply create a new smart playlist, with a rule to only include tracks added after the date of your backup (I backup up mine on 6/8/05). Only music added to your library after that date will be put into the playlist. The next time you want to make a backup, simply burn this playlist to CD/DVD, and then change the date in your rule to your new backup date.
Using iTunes to backup your music – [One digital life]