APA (7th ed.) Citation

Oppenheim, E. P. (2009). The Double Four. Project Gutenberg.

Chicago Style (17th ed.) Citation

Oppenheim, E. Phillips. The Double Four. Project Gutenberg, 2009.

MLA (9th ed.) Citation

Oppenheim, E. Phillips. The Double Four. Project Gutenberg, 2009.

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