APA (7th ed.) Citation

Elton, M. A., & Elton, C. I. (2006). The Great Book-Collectors. Project Gutenberg.

Chicago Style (17th ed.) Citation

Elton, Mary Augusta, and Charles Isaac Elton. The Great Book-Collectors. Project Gutenberg, 2006.

MLA (9th ed.) Citation

Elton, Mary Augusta, and Charles Isaac Elton. The Great Book-Collectors. Project Gutenberg, 2006.

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