Graham's Intelligent Investor - A History of Investing

In each revised edition of the Intelligent Investor, Graham discarded the formulas he presented in the previous edition and replaced them with new ones. He wasn't so wedded to investing ideas that he'd stick with them when too many other investors caught on to those theories. What changed was competition grew as opportunities became well known; technology made information more accessible. Since economies evolve, recent history is often the best guide to the future.