APA (7th ed.) Citation

Oppenheim, E. P. (1999). The Vanished Messenger. Project Gutenberg.

Chicago Style (17th ed.) Citation

Oppenheim, E. Phillips. The Vanished Messenger. Project Gutenberg, 1999.

MLA (9th ed.) Citation

Oppenheim, E. Phillips. The Vanished Messenger. Project Gutenberg, 1999.

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