The Intelligent Investor is definitely a worthwhile read. While I prefer the Random Walk approach to investing, Graham's book is unmistakably the best literature ever written on security analysis. Check out my blog for my other favorite reads! http://straightforwardfinance.blogspot.com/
I'm not sure , but i think it's business earnings (profit) divided by the value of the tangible assets of the business (book value). He appears to state that earnings were usually around 10% of the book value of the business. it gets a bit confusing when he starts talking about the difference between book and market values however i think the numbers work like this. With 10% profit (relative to book value) assuming an average dividend of 3.5% (relative to market value, * 1.6 to get book value rate of 5.4% for dividends paid out) leaving the remaining profit to be reinvested by the business a total increase in the book value of around 4%. Let me know what you think as i'm not sure myself.