The Outlook Address Book isn’t really an address book per se; it’s more of a service that lets you access all your contact sources in one place. For example, when you click To in a new mail message, Outlook pulls this list from your Outlook Address Book.

To get to the address book, click the Tools menu and click Address Book. (You can also use the keyboard shortcut Ctrl + Shift + B, or click the  icon on the toolbar.)