APA (7th ed.) Citation

Apel, W. (1958). Gregorian chant. Indiana University Press.

Chicago Style (17th ed.) Citation

Apel, Willi. Gregorian Chant. Bloomington: Indiana University Press, 1958.

MLA (9th ed.) Citation

Apel, Willi. Gregorian Chant. Indiana University Press, 1958.