APA (7th ed.) Citation

Butler, E. P. (1990). Pigs is pigs. Project Gutenberg.

Chicago Style (17th ed.) Citation

Butler, Ellis Parker. Pigs Is Pigs. Champaign, Ill.: Project Gutenberg, 1990.

MLA (9th ed.) Citation

Butler, Ellis Parker. Pigs Is Pigs. Project Gutenberg, 1990.