Calculus Made Easy: Being a very-simplest introduction to those beautiful methods which are generally called by the terrifying names of the Differential Calculus and the Integral Calculus

Read "Calculus Made Easy: Being a very-simplest introduction to those beautiful methods which are generally called by the terrifying names of the Differential Calculus and the Integral Calculus" by Thompson, Silvanus P. (Silvanus Phillips) for free. A classic public domain work covering calculus. Available in full text from Project Gutenberg.