The book I'm learning from: http://geometricalgebra.net
My Twitter: http://twitter.com/VinoBS
Errata:
17:40 This development of contraction product assumes that a and b are orthogonal to begin with. With two arbitrary vectors a and b that are not necessarily orthogonal, you don't have that a|X = b.
51:30 You'll see theta degrees of rotation if a and b are theta/2 degrees apart.
53:03 Hint: (ab)^-1 = b^-1 a^-1