there are two a very interesting references:
The Giroscopic Compass - The Theory of the Giroscopic Compass
The MacMillan Company - New York 1944
And there is also a paper by Ernest F. Barker (Univ. of Michigan) called "Elementary Analysis of the Giroscope". It seems to have been published in American Journal of Physics (not a recent issue, cause it was received on March 3, 1960).
This last reference is very interesting because it does not mention angular momentum and torque. The only physical reason it mentions is the unbalance of centripetal force from side to side.