Here are the steps to have your Outlook Address Book open your contacts by default.  You can also use these steps to choose any other address list you may have connected to Outlook.  This is particularly useful when Outlook has an Exchange account as it defaults to the Global Address List.

1. From the Home Tab on the Outlook Ribbon click on Address Book
   
2. Now the Address Book is Open click on Tools
3. Click on Options
4. Click on the drop down under “When opening the address book, show this address list first:
   
5. Choose the Contacts option and click on OK
6. If you like you can also click on the radio button which says “Start with contact folders” under “When sending email, check address lists in this order” if you wish Outlook to check your contacts folder first when searching for an email address if required.

Applies to: Outlook 2016, Outlook 2013, Outlook 2010