@ Junaid,

I think it is not a very difficult subject and any person can do it.Just by
studying British constitution you can get the initial idea about this subject.
Read the British constitution carefully and the remaining will not remain
difficult for you.
All constitutions are same in basics and yes in certain aspects they are
different but that doesn't mean thay are difficult ar out of reach.
Here are some books which i am using and i found them good and
sufficient for CSS preparation.
Here they are,

1.World constitutions by Kaeley.
2.How states are governed by Sohail Bhatti.
3.Constitutions of the world by Vishnoo Bhagwan and Vidya Bhushan.
4.Modern constitutions by Mazhar ul haq.
5.Constitutional development in Pakistan by G.w.Chaudhry.

I found these books really nice and these are what i have for this subject
beside some help from internet.
It is not time consuming subject as i am appearing in 2011 so i am preparing
it gradually with other subjects.Just preparing two constitutions in one month
and that's it.
Its objectives are quite easy and after studying all constitutions i am
sure you will also find objectives quite easy.Just prepare them side by side
and you will face no problem in this area.
And in the end i would say that man can do anything and if you study
with full devotion this subject will become the most easiest for you.
Wish you best of luck.

__________________
engr.saad