APA (7th ed.) Citation

Wollheim, D. A. (2010). The Secret of the Ninth Planet. Project Gutenberg.

Chicago Style (17th ed.) Citation

Wollheim, Donald Allen. The Secret of the Ninth Planet. Project Gutenberg, 2010.

MLA (9th ed.) Citation

Wollheim, Donald Allen. The Secret of the Ninth Planet. Project Gutenberg, 2010.

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