Book values have some good and some bad features.
Use book value as a starting point to try and establish the value of the enterprise.
Try to buy assets at a discount than to buy earnings.
Make sure you have the courage to stay true to your convictions and not let the market affect your emotions.
Before selling, try to re-evaluate the company again and see where the stock sells in realtion to its book value.
Graham liked the idea of protection on the downside.