I have at last brought out a book aimed at introducing people to geometric algebra  and geometric calculus.  Here is a pdf exhibiting the cover, the table of contents, and the preface:  An Introduction to Geometric Algebra and Geometric Calculus .

As indicated in the preface, the book was written specifically with advanced undergraduate and beginning graduate math majors in mind, but I hope it will be accessible to readers coming from other disciplines as well.

Here are links to a few sites where it can be purchased:  Barnes & Noble, Willdmansbooks, and Amazon.