In addition to gibsonf1's recommendations (not books), the following books are good. The first 2 are more pure introductions to the math, the third is applying it to physics, the fourth to CS.

Linear and Geometric Algebra, by Alan Macdonald: http://www.faculty.luther.edu/~macdonal/laga/

Vector and Geometric Calculus, by Alan Macdonald: http://www.faculty.luther.edu/~macdonal/vagc/index.html

Application to physics: New Foundations for Classical Mechanics by David Hestenes: https://books.google.com/books/about/New_Foundations_for_Cla...

Geometric Algebra For Computer Science by Dorst, Fontijne, and Mann: http://www.geometricalgebra.net/index.html

I highly recommend https://bivector.net/

In the docs section, the Geometric Algebra Primer by Jaap Suter is excellent. http://www.jaapsuter.com/geometric-algebra.pdf

A survey of geometric algebra and geometric calculus, A Macdonald - Advances in Applied Clifford Algebras, 2017 - Springer

edit: link http://www.astro.umd.edu/~jph/GAandGC.pdf

I'm currently going through Geometric Algebra for Physicists, and find it exceedingly clear:

https://g.co/kgs/ciKXVS

Clifford Algebras: An Introduction

Author(s): D. J. H. Garling Series: London Mathematical Society Student Texts 78 Publisher: Cambridge University Press, Year: 2011 ISBN: 1107096383