Hello , I am currently studying from the book Calculus - Volume 1 by Apostol . I am doing it as a pre cursor to studying Real Analysis by Rudin. I am currently on the second chapter of the book. I am solving the exercises as best as I can. My question is , while I am doing this book is it necessary to supplement it with a book such as 'Introduction to calculus and Analysis' by Courant or Spivak's Calculus. I usually don't face any difficulties in understanding the material from Apostol. What has happened is that there has been a 1 month gap since I studied it last. And I am unable to gain the flow which I initially had with the book. This has happened because sometimes I have to refresh some of the older concepts. Should I restart the book from the beginning to regain the flow or should I use Courant's book to do those parts ? Is it possible to approach Rudin after doing Apostol's book or one should also attempt Spivak/Courant before doing so?