Are you looking to make movie DVDs that play on regular DVD players? If so, keep in mind that iDVD does not work with external drives. Also, iDVD and DVD Studio Pro requires a G4 processor.
OK, with that said, the Formac Devideon is a Firewire DVD-R/RW that can be used with your iBook. It even comes with software for authoring DVDs (although I've heard it's not as good as iDVD).
That's pretty much your only choice.