APA (7th ed.) Citation

Twain, M. (2005). The Letters of Mark Twain. Project Gutenberg.

Chicago Style (17th ed.) Citation

Twain, Mark. The Letters of Mark Twain. Project Gutenberg, 2005.

MLA (9th ed.) Citation

Twain, Mark. The Letters of Mark Twain. Project Gutenberg, 2005.

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