Elementary Abstract Algebra, by W. E. Clark
Complex Analysis, by G. Cain
Complex Numbers, Quaternions, and Octonions for Everyone
Introduction to Quaternions, by P. MacLeod and D. Simpson
Quaternions, by M. J. Baker
Quaternion Algebra and Calculus, by D. Eberly
Quaternions and Rotations, by S. Gortler
Quaternion Interpolation (SLERP), by M. J. Baker
Quaternion Interpolation
Visualizing Quaternions (book homepage), by A. J. Hanson
The Octonionic Eigenvalue Problem, by T. Dray and C. A. Manogue
Finding Octonionic Eigenvectors Using Mathematica, by T. Dray and C. A. Manogue
Octonions, by J. C. Baez
Quaternion Fractal Animations: 1 , 2