One way to make a backup copy of your Address Book data is to choose Export from the File menu, and then Address Book (WAB) from the submenu. You'll be prompted for a filename. Browse to the folder or network share where you want to store your backup, supply a filename, and click Save.
Alternatively, you can simply make sure that your Address Book file is included in whatever backup procedure you routinely follow. You can find out the name and location of your Address Book file by choosing About from the Help menu.