BibTeX:
@incollection{RyyihKhnum1969,
  author = {Rúḥíyyih Khánum},
  title = {The Letter That Changed a Young Man's Life},
  booktitle = {The Priceless Pearl},
  year = {1969},
  publisher = {Bahá'í Publishing Trust},
}

APA:
Rúḥíyyih Khánum. (1969). The Letter That Changed a Young Man's Life. In *The Priceless Pearl*. Bahá'í Publishing Trust.

MLA:
Rúḥíyyih Khánum. "The Letter That Changed a Young Man's Life." *The Priceless Pearl*, Bahá'í Publishing Trust, 1969.

Chicago:
Rúḥíyyih Khánum. "The Letter That Changed a Young Man's Life." In *The Priceless Pearl*, Bahá'í Publishing Trust, 1969.