APA (7th ed.) Citation

(1964). Letters to John Glenn: P.S. I listened to your hertbeat. World Book Encyclopedia.

Chicago Style (17th ed.) Citation

Letters to John Glenn: P.S. I Listened to Your Hertbeat. World Book Encyclopedia, 1964.

MLA (8th ed.) Citation

Letters to John Glenn: P.S. I Listened to Your Hertbeat. World Book Encyclopedia, 1964.

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