It changes a person's perspective because it is introducing a new way to look and think about things. Even if it is in agreement with your current ideas,the author might have different reasons for seeing things his or her way, and so it opens up new ideas within the reader. So, how it changes it is by explaining new ideas through characters and their actions, or what the author says. When would be throughout the course of the book. In what depends on what book you are reading. If you believe in Creationism, and then read a book about evolution, your views are likely to change slightly, even if you are just sitting there thinking 'Oh that was stupid, but I can see why some faithless people might believe this way'. Their perspective still changed, even though their opinion did not.
It warned Roosevelt that Germany was developing an atomic bomb and encouraged him to start a nuclear program in the US that later became the Manhattan Project
