APA (7th ed.) Citation

Castle, A., & Castle, E. (1906). If youth but knew! (Copyright ed.). Tauchnitz.

Chicago Style (17th ed.) Citation

Castle, Agnes, and Egerton Castle. If Youth but Knew! Copyright ed. Leipzig: Tauchnitz, 1906.

MLA (9th ed.) Citation

Castle, Agnes, and Egerton Castle. If Youth but Knew! Copyright ed. Tauchnitz, 1906.

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