Sadly the PowerBook as well as the iBook has an older AirPort card which does not have an updated driver that offers WPA within either Tiger or Leopard OS's.
The newest version of OS-X either the PowerBook or iBook will support is ''*This model is capable of using Mac OS 9 applications within the Mac OS X "Classic" environment provided with Mac OS X 10.4.11 "Tiger" and lower ("Classic" is not supported starting with Mac OS X 10.5 "Leopard"). It cannot boot into Mac OS 9.'' per EveryMac.
Your only hope here is to find an external USB WiFi adapter which you can get with WPA support. You'll need to look at the used hardware on eBay to find one.