Sure. The book starts with a little girl in Norway during the 1980's. She lives with her mother and her father is in the UN away. One day she receives a letter containing questions and a philosophy reading from a person who calls himself Alberto Knox. They correspond for a while, Sophie learns about philosophy, when she receives a postcard saying happy birthday to someone else with the same birthday called Hilde. SPOILERS AHEAD SPOILERS
Alberto and Sophie meet, then Aberto tells Sophie that she is only a character in a book, entitled Sophie's World, on philosophy that is a birthday present. The viewpoint then switches the Hilde, who is reading about Sophie as Sophie figures out that she is a character in a book. Sophie then tries to break out, eventually becoming a spirit, unable to interact with Hilde and the real world.
SPOILERS Essentially, you're reading a book about a person reading a book about a person reading books about philosophy.