Do you know what aspect of drama you will be teaching?
You should start with theater history. A textbook that takes you from Greek theater to present day. Maybe "A History of the Theatre" by Oscar Brockett. (Although it's quite expensive, so see if you can find a used copy. There are also several other less expensive texts)
Theater Games for the Classroom by Viola Spolin (and she has other good books as well)
Respect for Acting and Challenge for the Actor by Uta Hagen
Introduction to Theatre Arts by Suzi Zimmerman
Technical Theater for Nontechnical People by Drew Campbell
Everything About Theatre by Robert Lee
If anyone ever tells you that you put too much Parmesan cheese on your pasta, stop talking to them. You don't need that kind of negativity in your life.