APA (7th ed.) Citation

Gaarder, J. (1991). Sophie's world: A novel about the history of philosophy. Berkley Books.

Chicago Style (17th ed.) Citation

Gaarder, Jostein. Sophie's World: A Novel About the History of Philosophy. New York: Berkley Books, 1991.

MLA (9th ed.) Citation

Gaarder, Jostein. Sophie's World: A Novel About the History of Philosophy. Berkley Books, 1991.