well about the books available in the market, especially 20 questions on indo pak the one by muktaba faridi, written by ghanzfar, is the best of the lot.
the guy actually accumulated the references and quotes from both k ali and i h qureshi and if u look at the css syllabus there is mentioning of studying each and evry bit of indian history
there are rather topics to be perfected by the aspirant
a person , no doubt can read the most lengthiest of a book ,if it is piece of literary art as well as history but he could stuff his mind with all the chronological data in the book
i had read gibbons decline and fall of roman empire in 6 to 7 days averaging 150 to 200 pagges or more in one day but i am not even 30 percent ready to take a paper of roman history i just got the idea of how the roman empire collapsed (recimer pretorian guard theodesus jolian jovian christian goths huns saracens legions ostro goths vandals turks taimoor and other) so use the 20 questions 30 questions but equip your self with all the necessary and impressive information on various topics dont be a rattoo of history u are needed to provide a critical assay of a related topic
for objective use engineer naveed sarwer objectives of indopak and use o level history of indopak for the chronology (may be some other book)