APA (7th ed.) Citation

Oppenheim, E. P. (2006). The Great Secret. Project Gutenberg.

Chicago Style (17th ed.) Citation

Oppenheim, E. Phillips. The Great Secret. Project Gutenberg, 2006.

MLA (9th ed.) Citation

Oppenheim, E. Phillips. The Great Secret. Project Gutenberg, 2006.

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