http://roguelikedeveloper.blogspot.com/2008/05/unangband-magic-system-part-one.html
(This article is in fifteen parts: parts one, two, three, four, five, six, seven, eight, nine, ten, eleven, twelve, thirteen, fourteen and fifteen, along with the follow up series Designing a Magic System Redux).
This is a fascinating and useful read… in 15 parts. 