BibTeX:
@incollection{ShoghiEffendi1982,
  author = {Shoghi Effendi},
  title = {Pray Assure American Believers},
  booktitle = {Bahíyyih Khánum: The Greatest Holy Leaf},
  year = {1982},
  publisher = {Bahá'í World Centre},
  url = {https://www.gutenberg.org/ebooks/19242},
}

APA:
Shoghi Effendi. (1982). Pray Assure American Believers. In *Bahíyyih Khánum: The Greatest Holy Leaf*. Bahá'í World Centre.

MLA:
Shoghi Effendi. "Pray Assure American Believers." *Bahíyyih Khánum: The Greatest Holy Leaf*, Bahá'í World Centre, 1982.

Chicago:
Shoghi Effendi. "Pray Assure American Believers." In *Bahíyyih Khánum: The Greatest Holy Leaf*, Bahá'í World Centre, 1982.