here's my little contribution
from my experience i would suggest
practice manual is useless as it does not cover whole content, in some chapters not even 30% of syllabus is covered
dinesh madan and others- these books present everything in lucid n easy language but they do not cover whole syllabus, but the far more important thing against these books is that they change/convert the language of headings, and in ISCA headings are the most important thing. esp. main headings, as there are many topics whose meaning is almost same but the points within those topics are very different. i am not able to recall any examples but believe me this is a big problem with these books.
I would recommend- read only study material, read it as many times as possible(3,4,5,6...) mug up headings and also remember the exact topic to which those heading belong.
M. Valecha can help you in remembring the headings but do not try to remember all the codes given as it will create more confusion as there are hundreds of small small topics in isca.
one more thing that valecha may help you to figure out is which questions are of 8 marks and which one can not be asked for more than 4 or 5 marks.
in every ques there first part is of 8 marks and the other 2 are of 4 marks.
give more stress in mugging up the headings of 8 marks questions.
ex. there are topics with 10 or more sub headings, these are the 8 marks question and those with 5 or7 or maybe 9 headings can be asked at most for 5 marks.
word of caution- do not depend fully on valecha as you can not survive on vitamin capsules one needs real food, here capsules is valecha's book (as only headings are given) and real food is study mat.
never ever start isca from chapter no. 3 which is Control objectives, this is the leanthiest and most difficult one, first complete IT act., ERP, BCP, 1st chapter, security policy. then complete SDLC and others. there is one more chapter, i think with "audit" in its name, this chapter relates to COBIT CoCo etc. leave this chapter, simply leave this chapter, i am writing it again, leave this chapter, this one is impossible to learn unless you are exceptionally good at mugging up.
if you dont have much time then in the chapter of control objectives you can leave the middle 50-60 pages which relates to' controls applicable on SDLC' and ends when topic of encryption start.
remember the sections of IT act.
try to remember the words used by study material for headings.
dont focus on 'learning by understanding' but simply 'learning by reading as many times as time may allow you'.
what i have come to understand is that our current president or BOS chairman or whoever is responsible for setting the papers are very rational people, they are focusing on concepts and understanding in subjects like DT, Cost, Law etc. in ISCA straight questions are asked but what gets confusing is remembring the sub headings of the asked question and for that there is only one technique ie. mug the S*** out of ISCA.