Any value investor will tell you that the best time to buy a stock is when it is trading below its intrinsic value. But how does one arrive at the intrinsic value of a company? There are different ways to arrive at a company's worth. One of them is to look at the book value of a company. A company trading at a price less than the net book value is considered to be attractively priced. Looking at price to book value is a very useful metric, as the market can be inefficient at times and ignore companies that are actually trading at less than their net worth.