Well, the answer is that NCERT books are completely accomplished in their way, and CBSE will ask little more questions from these books. Students should read and study these books in detail. As far as side books are concerned, it doesn't harm to study from them, but then you should be more than 100% sure that you have covered all the lines of NCERT. Before switching to other books, study NCERT books thoroughly. NCERT books will provide you a better understanding of the concepts rather than other books. You can choose side books as an option, but NCERT should always remain your first choice.