BibTeX:
@incollection{RyyihKhnum1969,
  author = {Rúḥíyyih Khánum},
  title = {Nothing but the Best: Shoghi Effendi and Excellence as Worship},
  booktitle = {The Priceless Pearl},
  year = {1969},
  publisher = {Bahá'í Publishing Trust},
  url = {https://bahai-library.com/khanum_priceless_pearl},
}

APA:
Rúḥíyyih Khánum. (1969). Nothing but the Best: Shoghi Effendi and Excellence as Worship. In *The Priceless Pearl*. Bahá'í Publishing Trust.

MLA:
Rúḥíyyih Khánum. "Nothing but the Best: Shoghi Effendi and Excellence as Worship." *The Priceless Pearl*, Bahá'í Publishing Trust, 1969.

Chicago:
Rúḥíyyih Khánum. "Nothing but the Best: Shoghi Effendi and Excellence as Worship." In *The Priceless Pearl*, Bahá'í Publishing Trust, 1969.