Data Validation using VBA to reference to another book
 

I have got 2 workbook. book1.xls is for application and book2.xls is a database that looks like this

A B
Name1 CompanyA Address
Name2 CompanyB Address
Name3 Company C Address

In book1.xls cell B1, I want to create a drop down list (data validation) making reference to book2.xls column A, i.e. when cellB1 is focus, I will see Name1, Name2, Name3 in the drop down list. How can I achieve this