It depends on the car mount. The BookBook shell has a metal plate under the leather that sticks to strong magnets in the back of the wallet portion of the BookBook. So, if your car mount has magnets that are strong enough to hold the weight of the iPhone through the leather-covered metal plate in the shell, it should be compatible.
Note that the MagSafe magnets in the BookBook shell connect the iPhone to a MagSafe charger, but they are not strong enough to hold the weight of an iPhone to a charger in a moving car.