Sorry for the long delay...
Thanks for the suggestion. Yes, I did try repairing permissions, it had no effect.
I did look in the System log & found the following erors generated when I attempt to start the app:
Feb 25 15:50:15 G5Mac Address Book[1287]: Can't read (null) error
Feb 25 15:50:15 G5Mac com.apple.launchd[851] ([0x0-0xa50a5].com.apple.AddressBook[1287]): Exited with exit code: 1
Even though the system is 10.5.8, I tried (re) installing "MACOSXUpdComb.dmg". Again, no effect whatsoever.
Another strange thing I've noticed; the dock icon is not what I'm used to seeing... it looks like this:
Whereas I'm used to seeing this:
Starting to fear I'll have to reinstall from scratch & update. (shriek!)
Thanks...