Visual Complex Analysis - If you already know the basics of complex numbers but want to get an in depth understanding using an geometric and intuitive approach then this is a very good book. The book explains how to represent complex transformations such as the Möbius transformations. It also shows how complex functions can be differentiated and integrated.
Complex Numbers
Visualizing Quaternions by Andrew J. Hanson
Quaternions and Rotation Sequences.
On Quaternions and Octonions
