Cita APA (7th ed.)

Butterfiel, W. (1964). Effective personal letters (2nd rev.). Prentice Hall.

Cita Chicago (17th ed.)

Butterfiel, W. Effective Personal Letters. 2nd rev. Englewood Cliffs: Prentice Hall, 1964.

Cita MLA (9th ed.)

Butterfiel, W. Effective Personal Letters. 2nd rev. Prentice Hall, 1964.