Hi, this has been discussed in depth several times... you can search for it, or I can also give you a synopsis: OS X is not really designed for application-level password protection. For lots of reasons, most of them fairly good, and rather better than the alternatives.
Here are your major options for getting security out of Address Book:
1) Secure your computer properly -- make sure you have a login password, that requiring the password at login is not disabled, and that you have set the computer to screen-save relatively quickly and require a password when waking from sleep or screensave. You can also set a hot corner or do several other things to facilitate using screensaver to lock the computer, or you can enable fast user switching and switch to the login window.
2) Create a secondary "secure" account in which you keep privileged information and FUS to it when you need this information. Or alternately create a secondary "guest" account and FUS to it when people other than you need to access the computer.
As far as password protected alternatives go, I'm not sure. Sorry.
