I dont think it is necessary to have both, dealers use monthly "grey sheets" with up to date values based off of collectability and the market value of the metal. Bluebooks have dealer prices. Dealer prices will be lower than the market value. Its what you can expect to receive from a sale of your coin. Good dealers will likely sell their coins for those same prisces. There are plenty of online databases that can provide up to date values that you can expect to pay at reputable dealers. Just look for the ANA oil lamp on the store front window.