There is a very rich and interesting project that walks through the concepts of the Golden Mean and the ideas of proportion. Robert Charlton puts together the site from selections from "Looking and Seeing 3 - THE SHAPES WE NEED" by Kurt Rowland ©1965 (out of publication).

Presented in a small, nicely proportioned window, this is a great primer to understand the ideas of proportion and the base mathematics behind the numbers as well as how these ideas can be seen in nature and how these ideas have been used in architecture