I really liked these two books (english) :

Towards understanding Islam by Abū'l-Aʿlā Mawdūdī
Very concise book with excellent logical arguments on different aspects of Islam, like, why is there a need for a religion. Mawdudi sahib are famous for their great arguments. BUT Only 1st, 4th and 5th chapter are relevant for the CSS exam. For the rest of the syllabus areas, Dogger publisher's book on islamiat is okay.

I have Mawdudi sahib's book in MS word format, how can i share it with you here?