APA (7th ed.) Citation

Doyle, A. C. (2005). The Sign of the Four. Project Gutenberg.

Chicago Style (17th ed.) Citation

Doyle, Arthur Conan. The Sign of the Four. Project Gutenberg, 2005.

MLA (9th ed.) Citation

Doyle, Arthur Conan. The Sign of the Four. Project Gutenberg, 2005.

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