Here is the best article I've found on Quaternions to date:
Minkowski, Quaternions, Complex Numbers:
more to it than meets the i
by Tim Shelton-Jones
[edit]
The above article actually concerns Communative Quaternions, which are essentially hacked quaternions who now obey the communative property (a*b = b*a). Imagine communative cross-products. Of much greater use is the link below, which discusses actual implementation in OpenGL, my API of choice.
Delphi3D and Quaternions
Although I'm coding in C++ and not Delphi, the ideas and API are identical.