Engineering books are very expensive in my country. I want to give calculus a spin. Spivak is a hundred dollars.
I did chemical engineering long ago (how long? Computerized process control was just being introduced into the curriculum, and we learned to program in Fortran). I did several calculus courses, but it was always just a matter of memorizing techniques.
Much later, I came across Calculus Made Easy on Project Gutenberg, a textbook from 1914 that actually helped me to understand why calculus works, instead of just treating it like magic.