APA (7th ed.) Citation

Chesterton, G. K. (2007). The Man Who Knew Too Much. Project Gutenberg.

Chicago Style (17th ed.) Citation

Chesterton, G. K. The Man Who Knew Too Much. Project Gutenberg, 2007.

MLA (9th ed.) Citation

Chesterton, G. K. The Man Who Knew Too Much. Project Gutenberg, 2007.

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