According to me as well as several toppers of previous IAS exams, the best way to start your UPSC preparation is by reading through the NCERT books for UPSC and then moving on to the standard IAS book list (consisting of the standard books).
Also, you should be mindful about the type of IAS books that you choose for your preparation, given that the UPSC prelims and UPSC mains are very different from each other and would need 2 different sets of reading material for each.
