Sophie's World A Novel about the History of Philosophy

One day when Sophie comes home from school, she finds two questions in her mail: Who are you? and Where does the world come from? Before she knows it, she is pondering all the great questions of Western philosophy (from the Greeks to Kant, to Marx and Freud) with a mysterious mentor. But Sophie is also receiving a separate batch of equally unusual letters. Who is Hilde? And why does her mail keep turning up in Sophie's world? To solve this riddle, Sophie uses her new knowledge of philosophy, but the truth is far stranger than she could have imagined.