To open an existing address book use the menu command File/Open... or choose an address book from the File/Recent list.
The File/Open... menu command will allow you to open any previously created address book. Address books are stored with the filename extension of .abk.
The File/Recent menu will list up to 6 of the most recently opened address books. Select any of these address books to quickly open them.
About Address Books
Creating a New Address Book
Saving an Address Book