Why When the bus stops suddenly, the book slides forward off the seat ? (You put your book on the bus seat next to you. )?

Because of Newt's First Law of Motion. A body in motion tends to stay in motion.... And when that bus puts on the brakes, the book tends to stay in motion. So the book slides forward because it tends to keep on moving while the bus is stopping.

Forward momentum. It's a physics thing. You sort of slide forward too but not nearly as much because you weigh a lot more than book. But given enough speed you could fly through the windshield right after a sudden stop.

The books don't have brakes to stop the forward motion.