The book falls over because there is no net force acting on the book.
To solve this question, we need to understand the concept of Newton's Third Law of Motion.
Newton's Third Law of Motion states that for every action there is always an equal but an opposite reaction.
When the brake is suddenly applied to make the bus stop, the book will react to it but in an opposite direction based on Newton's Third Law of Motion.
Therefore, we can conclude that the book keeps moving forward when the brake was applied to the bus that was why it falls over and since there is no net force to cancel out the reaction produced by the brake of the bus.
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