No problem at all.
• Connect the disk to the iBook via FireWire like you mentioned. Make sure it is formatted and give it a name (you can do that with Applications/Utilities/DiskUtility)
• Turn on "Personal File Sharing" on the iBook (in System Preferences > Sharing)
• Note the name of the iBook above ("Computer Name")
• On another Mac, go to the Finder and chose Go > Connect to Server...
• Enter afp://iBookName.local, select "Registered User", and enter your user name and password
• Select the external disk connected to the iBook
In the future you should be able to mount the disk by going to Apple menu > Recent Items > Servers. You can also make an alias of the external disk and double-click it to mount it on any local Mac.
As long as the disk isn't disconnected form the iBook and the iBook is running (with file sharing turned on) you should be able to access it form every Mac on your wireless network (hence the ".local" in the afp address). Of course this will not work from outside your wireless network which is probably what you want too (security).
Let us know how that works out for you. Good luck.
