Try to find the papers from Ken Shoemake. He has an article in the book Graphics Gems IV ( ArcBall, Section III.1 ).

A long time ago I wrote a STL viewer that utilizes quaternions to compute the model rotations using what I learnt from those papers.

https://github.com/jimijimi/trekanter_mingw

Warning: Very UGLY code, so be easy on me( win32 cross compiled on Linux, ugh! ).