APA (7th ed.) Citation

Ball, R. S. (2000). Great Astronomers. Project Gutenberg.

Chicago Style (17th ed.) Citation

Ball, Robert S. Great Astronomers. Project Gutenberg, 2000.

MLA (9th ed.) Citation

Ball, Robert S. Great Astronomers. Project Gutenberg, 2000.

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