actually, i don't think you have to believe to understand. i don't believe but at least i have the illusion that i understand.
we have a basic law in the constitution. situations come up all the time which require rulings based on the constitution and on legal precedents. (this comparison stinks in that the constitution can - and has been - changed and ammended, whereas in religious law the basics stay the same.)
having read most of the book that was the subject of this thread - it is exactly as the title suggests - the evolution of islamic jurisprudence, the reasons for the development of the 4 main schools of thought, the effects of outside circumstances and the reasons for the decline and stagnation of scholarship and the arising of blind following. (haven't quite finished it yet). so it talks about how islamic jurisprudence changed and developed over time and the reasons for the changes.
this does not imply that the 'basic general plan' is imperfect. just that the law is living.
islam and judaism are quite different from christianity in that they are entire systems of law. as new challenges arise, it is for scholars to see which parts of the "basic general plan" are applicable and it what way. sometimes this has to do with very small mundane things that arise in daily life and seem completely absurd to the outsider.
one cool advantage of all this is that it makes the believer consciously aware of God as he goes about his daily life.
the book is quite an interesting read and the print is nice and big. and it's not too long either.