APA (7th ed.) Citation

Oppenheim, E. P. (2012). Mr. Marx's Secret. Project Gutenberg.

Chicago Style (17th ed.) Citation

Oppenheim, E. Phillips. Mr. Marx's Secret. Project Gutenberg, 2012.

MLA (9th ed.) Citation

Oppenheim, E. Phillips. Mr. Marx's Secret. Project Gutenberg, 2012.

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