APA (7th ed.) Citation

Groner, A., & Colbron, G. I. (1999). The Case of the Registered Letter. Project Gutenberg.

Chicago Style (17th ed.) Citation

Groner, Auguste, and Grace Isabel Colbron. The Case of the Registered Letter. Project Gutenberg, 1999.

MLA (9th ed.) Citation

Groner, Auguste, and Grace Isabel Colbron. The Case of the Registered Letter. Project Gutenberg, 1999.

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