I am currently reading Calculus Volume 1 by Tom Apostol, I find it very good but hard. I often get stuck and it took me very long time to move from page to page (mostly pages with theorems). I am done reading till half of chapter 2. I just want to know if reading apostol is really a MUST for theoretical physics (NOT going to string theory) or can I resort to Calculus by Salas which I think is rigorous although not as rigorous as apostol. Any suggestions?