Boston Gear offers an overview of gear theory in the form of a free 5.5MB PDF file.
From the gear theory download page
This engineering information explains gear nomenclature, tooth formulas, backlash, tooth strength, torque and horsepower requirements and other standard gear selection formulas. Our Gear Theory Manual is a must for anyone who frequently deals with gearing in mechanical components.
Here's the link to the free gear theory PDF courtesy of Boston Gear.