Yes, you can do that and no, nothing will be affected on the mounted drive unless you choose to do so!
However you should pay particular attention to the section
"Tips for using target disk mode with Intel-based Mac computers"
Also since you don't say which version of OSX in on the PowerBook, note the comments regarding 10.3.9 and earlier together with the related article:
Yes, you can, but with some notes. Braby sent some data, I add some other.
Target Mode has two basic roles:
1. to turn a mac into an external drive and/or external optical drive (if it has one, until Air era, all had); you may use this way if your internal optical drive is damaged and you wish to use the optical unit of the other mac; or use it as a simple external disk for backup purposes or else.
2. to boot from a such-booted mac.
#1 is usable with any two macs disregarding whether they are both PPC, both intel, or combination PPC-intel, as the behavior is like any external disk drive or external optical drive.
#2 is impossible if you want to boot the other mac, they must be both intel or PPC, as you cannot use an intel-based mac as a boot mac for the other ppc-mac, and vice-versa.
So said and clarified, you may use, but not if you intend to boot the other one if you wish to use it for various purposes implying boot.