What you are seeing when you start to type in a name is not from the Address Book (AKA Contacts) at all but is an autocomplete function based on previous emails. To see the full list (and be able to remove old, unused entries) go to Window-->Previous Recipients. To bring up the Address Book/Contacts from within Mail click on the silhouette symbol button ("Show Addresses") or go to Window-->Address Panel or use the keyboard shortcut Option-Command-A. From this panel you can start new messages to any person, persons, or groups.