Go through syllabus and past papers of compulsory and optional subjects. And read rules and regulations regarding CSS. You can find syllabus and rules at the top right corner of this page. And past papers in the relevant sections of this forum.
There is a whole section dedicated for subject selection and for suggested books that which books are good for every subject. And then you can search in different threads about best books that different people used for exams.
For GK,
Trek to Pakistan and Ikram rabbani for Pak Affairs
Hafiz Karam Dad Chugtai for Islamiat
Mian shafiq for general science section. And for ability section you will have to go through different books related to GAT and various publishers have now published books on this portion.
And for general reading. Well, it's better to read opinion pages of any good English newspaper. Be it Dawn or The nation or Tribune.

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''Sufficient for us is Allah , and He is the best Disposer of affairs" Al-Quran