Geometric Algebra for Physicists - This is intended for physicists so it soon gets onto relativity, spacetime, electrodynamcs, quantum theory, etc. However the introduction to Geometric Algebra and classical mechanics is reasonable.
Geometric Algebra for Computer Science: An Object-oriented Approach to Geometry. This book stresses the Geometry in Geometric Algebra, although it is still very mathematically orientated. Programmers using this book will need to have a lot of mathematical knowledge. Its good to have a Geometric Algebra book aimed at computer scientists rather than physicists. There is more information about this book here.
Clifford Algebras and Spinors (London Mathematical Society Lecture Note S.)
Pertti Lounesto. This is very complex subject matter for graduates or undergraduates, however there is a lot of explanations and it is not all proofs like some mathematical textbooks. The book has a lot of information about the relationship between Clifford Algebras and Hypercomplex Algebras, I don't think I'm left with a full understanding of spinors though.
On Quaternions and Octonions
Quaternions and Rotation Sequences.
Visualizing Quaternions by Andrew J. Hanson
