An IAS officer is perceived to have ample and sufficient knowledge to deal with any sorts of problems ranging from daily chores to peculiar problems. He/She should be well l versed with all possible subject matters. Hence, in light of all these, it can be concluded that an elementary level knowledge pertaining to all themes and substances should be imbibed in an IAS officer. Therefore, subject matters of secondary level should from the foundation of an IAS aspirant.
Realistically specifying, there can never be a fixed number of prescribed books. Uniformity can never be maintained at least in mentioning the number of books to be referred. Some likes to read from internet, some likes to refer coaching institutes, notes, even some would like to prescribe only standard NCERT books and some may go for amalgamation of all these sources.
What all books, which all sources and how many in number solely depends upon the comfort and ease of candidates. These vary from time to time from aspirant to aspirant and from experienced to naïve candidates.