Exponentials Made Easy

Free

M. E. J. Gheury de Bray's 1921 sequel to Calculus Made Easy — written by a scientist who read Thompson's classic and set out to do the same for the subject it skipped. Rebuilt from the ground up for the web: powers, logarithms, log scales, and the one number that grows itself, told plainly with every curve a live, interactive animation. Public domain source; free to read.