The book value of a car is basically an estimate of what a car with certain characteristics should be worth. The value that any specific vehicle is actually worth is simply whatever someone is willing to pay for it, because at that moment in time, that vehicle was deemed to be worth that amount to the buyer. However, if the buyer pays an amount that is significantly higher than what other buyers from the same demographic area typically have paid for cars with similar characteristics, then the buyer has probably overpaid, which is a situation to try to avoid if you are the buyer (although this is probably a good thing if you are the seller).
While it may be interesting to know the book value when you are buying a new car, it is absolutely vital to have a good grasp on the value before you buy or sell a used car. If you are planning to buy a used car, then you should determine the book value of that car before you begin negotiations with the seller. That way you will be in a position to make a good counter offer. I would recommend giving a counter offer that is lower than the book value so that you will have some room to maneuever as the buyer will will probably try to get you to pay a higher amount. It is very important in this situation to not overpay for the vehicle. Staying around or under the estimated value of the car obviously will save your hard earned money but it is also very important if you are planning on financing the vehicle. Banks will typically only loan an amount up to a certain percentage of the vehicle's book value, so if you agree to buy a vehicle at a price higher than that amount, you will need to come up with a large enough down payment to at least cover the difference.
It is also important to have an accurate view of the book value if you are the seller. For sellers, I would recommend setting an asking price that is above the book value for the vehicle so that you have some room to give when the buyer tries to negotiate a lower price. when determining the vehicle's value for this purpose, be sure to account for any and all features of the car that increase its value and factor in your view of the condition of the vehicle.
used car book value calculator