Getting between your dog and visitors IS a positive method. It shows her that you've got the situation under control. It's not punishment.
Joy can give you some titles of good books.
In the meantime, why don't you check out this website?
www.clickersolutions.com
You'll want to browse some of the articles on "Basic Clicker Training". Then scroll down to "Problem Behavior". There are several articles on aggression and barking.