The "blue book value" of a used car is a reference to "Kelley Blue Book." Published since the 1920s, the "Kelley Blue Book" has long been the go-to source to determine the value of a used car. You can even use the VIN, or vehicle identification number, of a car to determine the Blue Book value. These numbers, which have been assigned to every individual car since 1980, were standardized to keep people from trying to sell a car as one make and model...