I would like to ask for some intermediate level textbook for calculus (single variable), or, at least, some supplement to Spivak's Calculus for better understanding on how to approach and solve his problems.
I read and done a lot of Stewarts and Leithold Calculus and was REALLY easy and boring. They arent so rigorous and sophisticated mathematically; the exercises are extremely easy. So I went straight to Spivak's Calculus, because everyone say it's rigorous. Indeed, it was TOO rigorous, haha. No problems until the limits section, nor in the theory of limits. I was enjoying it. However, I have huge difficult in solving the problems in this chapter, specially because there aren't many examples (at least, not in the same level of difficult of the proposed problems).
I have a good background in high-school math (Theory of Sets, Functions, Trigonometry, Polynomials, Complex Numbers, Analytic Geometry, Combinatorics, Basic Linear Algebra, Number Theory etc.).