Two easy ways to do it:
1. Move your entire "iTunes" folder over to the MacBook Pro. This works wether the folder is on the iBook's internal drive or the external drive.
2. If your iTunes library is currently on the internal drive on the iBook, use the Migration Assistant (you will be prompted to run it during first boot of the new computer,) to move all of your files from the iBook to the MacBook Pro via FireWire. (You will put the iBook into "Target Disk Mode", and the assistant will handle copying everything, including your music.)
Either way, the first time you try to play a purchased track on the MacBook Pro, it will ask you for your iTunes Store login and password to authorize the MacBook Pro.
One thing to be sure of is if you are going to get rid of the iBook, be sure to "Deauthorize" it first. To do so, open iTunes, go to the "Store" menu, and choose "Deauthorize Computer..." This is because Apple only allows purchased (DRMed) music to be playable on 5 computers, so you want to make sure to remove old computers so you don't end up with one of your authorizations "in use" by a computer you don't use.