Beware: Apple Address Book died on me, and ate my data. Yes, I keep a
backup; you should too.

When it was coughing and wheezing, it seemed to lock up applications
that depend on Address Book, including Mail and Adium. When Address
Book croaked, it took out my Mail previous-recipients list.

The master data lives in:
~/Library/Application Support/AddressBook

That's probably where you'll want to restore your data to, if this
happens.