BibTeX:
@incollection{theBb,
  author = {the Báb},
  title = {The Báb's Summons to the Kings},
  booktitle = {Selections from the Writings of the Báb},
  publisher = {Bahá'í World Centre},
  url = {https://www.gutenberg.org/ebooks/18828},
}

APA:
the Báb. The Báb's Summons to the Kings. In *Selections from the Writings of the Báb*. Bahá'í World Centre.

MLA:
the Báb. "The Báb's Summons to the Kings." *Selections from the Writings of the Báb*, Bahá'í World Centre, n.d..

Chicago:
the Báb. "The Báb's Summons to the Kings." In *Selections from the Writings of the Báb*, Bahá'í World Centre, n.d..