APA (7th ed.) Citation

Simak, C. D., & Gaughan, J. (2010). The World That Couldn't Be. Project Gutenberg.

Chicago Style (17th ed.) Citation

Simak, Clifford D., and Jack Gaughan. The World That Couldn't Be. Project Gutenberg, 2010.

MLA (9th ed.) Citation

Simak, Clifford D., and Jack Gaughan. The World That Couldn't Be. Project Gutenberg, 2010.

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