APA (7th ed.) Citation

Cecelia Ahern. (2007). P.S. I Love You. Harper.

Chicago Style (17th ed.) Citation

Cecelia Ahern. P.S. I Love You. London: Harper, 2007.

MLA (9th ed.) Citation

Cecelia Ahern. P.S. I Love You. Harper, 2007.

Warning: These citations may not always be 100% accurate.