The theory of the motor design is covered in depth here: http://www.allaboutcircuits.com/vol_2/chpt_13/5.html
(most modern steppers are the hybrid sort with neodymium magnets - older motors have ferrite magnets
and are a lot less torquey for the same size)
If you want high-performance (fast speeds) then you need to go to chopper-driven low-inductance bipolar stepper
motors - a lot of these online tutorials don't cover this, note. Unipolar motors with low voltage drivers are limited
to a few hundred rpm or so.