Islamic Studies is the easiest and simplest subject. Just try to be a traditional maulvi sahab, present your statistics facts and figures from history, justify your solid arguments and use colourful markers. Try to give more and more refernces with your logic and common sense in accordance with the said question. I belive one can easily pass this subject with flying colours. I had got commendable marks in my first attempt, it is the most easiest paper in CSS exam. To be very honest, I did not even prepared a single word from any book, I just attempted with my background knowldge and I knew one of my friends words who used to suggest me about Islamiat prior to exam "Beta Maolvi sahab ko impress karnay kay liye maolvi banjae aap" It means you need to be a pure Islamic oriented one. Try to be very simple and your presentation needs to be very simple and clear so that checker can award you good score. In addition to that, do not be in favour of any particular school of sect/thought, try to argue and justify your answer comparatively.

For simple wording book, just read answer hashmi's book and watch peace tv. This is more than enough to pass Islamiat. Howeiver, for contemporary issues, you need to google and search your topics on interment. I hope it would help you.

Jazak'allah.