I've been hunting around for information on quaternions, but I haven't found what I really wanted to know. Most people just say, "This works, and sometimes it's computationally faster." but they rely on matrices to calculate the rotation they want.
What I'd like is sufficient intuition for how quaternion rotations work in 4D so I can construct the rotation I want without having to know anything about matrices. Eventually, I figured out what I wanted to know myself, and wrote it up. (Now in its second draft.)
If anyone here would like to offer any constructive "peer review", it would be much appreciated. You can find a 285K pdf here: http://www.megaseattle.com/james/Geometry/Intuitive_Quaternions.pdf
(Edit: A new URL, and a new draft of the paper. Now with matrix conversions and more pictures!)
And many thanks to Andy Southgate for the article "Rotations in four spatial dimensions" (another pdf, available in the Mushware archives.) I found it more useful than most things out there.