Answer:
When the car is stopped at the traffic light, the dashboard (which is part of the car) and the book are both at rest. As the car accelerates forward, the dashboard moves forward. But no force is exerted directly on the book, so it remains at rest. Because of its inertia, the book does not move forward when the dashboard moves forward. From the reference point of the car, the book appears to move backward, sliding off the dashboard.
