APA (7th ed.) Citation

Riordan, R. (2011). Heroes of Olympus: The Mark of Athena. Penguin Group.

Chicago Style (17th ed.) Citation

Riordan, Rick. Heroes of Olympus: The Mark of Athena. London, England: Penguin Group, 2011.

MLA (9th ed.) Citation

Riordan, Rick. Heroes of Olympus: The Mark of Athena. Penguin Group, 2011.

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