Try: On G5: Launch Internet Connect.
Set Configuration pull-down to Airport (*)
(If "Airport Power" isn't "On" click "Turn Airport On")
Set Network pull-down to Create Network...
Password is optional but a good idea.
Click OK.
On iBook: Set Configuration pull-down to Airport (*)
(If "Airport Power" isn't "On" click "Turn Airport On")
Set Network pull-down to name of your G5 (which should appear in the pull-down.) If you set a password on the G5 create Network, enter that.
(*) If Airport doesn't appear in the Configuration pull-down, Launch System Preferences, go to Network pane, set "Show:" pull-down to "Network Port Configuration", make sure the line for "Airport" has a check-mark to enable it.
Then on G5: turn Personal File Sharing on (G5: System Preferences/Sharing pane/Services tab).
On the iBook: use Finder's Go>Connect to Server menu and the G5 should appear in list.
(Log in using the userID and password for the G5.)
OS X's help center should provide instructions if you type "create network" and "file sharing" as search terms.