You can transfer data/folders through FireWire target disk mode or over the network with a wired/wireless connection. You'd want to connect both computers with a FireWire cable and start up the iBook with the 'T' key held down. The iBook's hard drive will then appear on the desktop of the iMac and you can access all of the folders within it. Alternatively enable file sharing in System Preferences on the iBook, make sure both computers are on the same network, and view your local computer network from the iMac. If the iBook is visible, open it up and connect as a registered user using the name/password used to secure the iBook and drag the files over through that.
P.S. On second thought if you have iTunes 7 or later you can backup your library to multiple cds/dvds and when you insert it into the iMac it'll ask to bring that data into iTunes.