Found in 3 comments on Hacker News
riemannzeta · 2015-05-07 · Original thread
Although not a small amount of effort, it really helps to learn geometric algebra notation:

http://www.amazon.com/Geometric-Algebra-Physicists-Chris-Dor...

Once you get an intuitive grasp of bivectors, it's not hard to "see" tensors as a linear map of bivector into bivector.

happy4crazy · 2011-04-13 · Original thread
If you enjoy playing around with quaternions, you may also have fun with geometric algebra.

Geometric Algebra for Physicists is very clear and gently paced. http://www.amazon.com/Geometric-Algebra-Physicists-Chris-Dor...

This one also looks good: http://www.amazon.com/Linear-Geometric-Algebra-Alan-Macdonal...

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