Many people advise that we should learn concepts from only one book as using too many books lead to confusion. I have Spivak, Apostol and Kuratowski for Rigorous calculus. When I first read Apostol, I had a hard time understanding. After doing some problems in Spivak, I find the same in apostol easy. For example, Apostol first introduces integration but I could not follow the proof part before. Now, able to read and do proofs, I am able to follow Apostol. My primary aim is to solve Spivak and read Apostol or other books just for insight. Will such an approach be useful? I feel using different books give multi-intuitive feel.