APA (7th ed.) Citation

Luther, M. (1994). Luther's little instruction book (the small catechism of Martin Luther). Project Gutenberg.

Chicago Style (17th ed.) Citation

Luther, Martin. Luther's Little Instruction Book (the Small Catechism of Martin Luther). Champaign, Ill: Project Gutenberg, 1994.

MLA (9th ed.) Citation

Luther, Martin. Luther's Little Instruction Book (the Small Catechism of Martin Luther). Project Gutenberg, 1994.