TITLE: Spivak or Apostol?
QUESTION [6 upvotes]: Which one of those is the best for a person interested in pure mathematics and who wants to have a deep understanding of calculus? Apostol or Spivak? Could you guys tell me some differences between the approaches of them? What about the exercises? I would like to be challenged, but in a constructive way.

REPLY [2 votes]: I started with abbott Understanding Analysis then move on to Apostol. Stewart don't provide enough rigour but a good starting point for very early beginners into calculus. In my order
(1) Stewart calculus
(2) abbott Understanding Analysis (pretty small text)
(3) Apostol calculus vol 1
(4) Spivak calculus (for more understanding)
(5) Apostol vol 2