When I still used OS 9 in like 2000... I set it up on an external drive from my powerbook. This was a G3 Book though. I'm not sure if you 'Book allows for dual booting... I also used separate Partitions and installed that way. Newer Mac's aren't supposed to Boot OS9, I guess it would be dependant on the machine.
Having OS X and OS 9 in separate hard drives (ideally) or at least in separate partitions provides additional boot options. One option I do use frequently is holding down the OPTION key during boot-up. This allows you to change OS's during the early stages of startup rather than after it fully boots. If both OS's are in the same hard drive or partition you can only startup from the last OS used.
I'm not sure if your PowerBook model requires OS 9 installation from the restore disks or if you can install from the retail disks. The restore install procedure will only install OS 9 in the same partition as OS X, but after installation you can simply copy the OS 9 System Folder into your OS 9 designated partition and then delete it from your OS X partition. Then setup the OS X Classic preferences to startup OS 9 from your OS 9 partition.
If you can install 9.2.2 from the retail disks you still have to designate the Classic preferences.